Semiconductor device and manufacturing method thereof

ABSTRACT

An object is to provide a semiconductor device having a structure in which parasitic capacitance between wirings can be efficiently reduced. In a bottom gate thin film transistor using an oxide semiconductor layer, an oxide insulating layer used as a channel protection layer is formed above and in contact with part of the oxide semiconductor layer overlapping with a gate electrode layer, and at the same time an oxide insulating layer covering a peripheral portion (including a side surface) of the stacked oxide semiconductor layer is formed. Further, a source electrode layer and a drain electrode layer are formed in a manner such that they do not overlap with the channel protection layer. Thus, a structure in which an insulating layer over the source electrode layer and the drain electrode layer is in contact with the oxide semiconductor layer is provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/121,708, filed Sep. 5, 2018, now allowed, which is a continuation ofU.S. application Ser. No. 15/368,984, filed Dec. 5, 2016, now U.S. Pat.No. 10,079,306, which is a continuation of U.S. application Ser. No.14/848,492, filed Sep. 9, 2015, now U.S. Pat. No. 9,515,192, which is acontinuation of U.S. application Ser. No. 14/333,681, filed Jul. 17,2014, now U.S. Pat. No. 9,142,570, which is a continuation of U.S.application Ser. No. 13/784,908, filed Mar. 5, 2013, now U.S. Pat. No.8,809,856, which is a continuation of U.S. application Ser. No.13/717,806, filed Dec. 18, 2012, now U.S. Pat. No. 8,772,093, which is adivisional of U.S. application Ser. No. 12/846,603, filed Jul. 29, 2010,now U.S. Pat. No. 8,384,079, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2009-180077 on Jul.31, 2009, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device using an oxidesemiconductor and a manufacturing method thereof

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and electronic equipmentare all semiconductor devices.

BACKGROUND ART

In recent years, much attention has been directed to a technique forforming a thin film transistor (TFT) by using a semiconductor thin film(with a thickness of approximately several nanometers to several hundrednanometers) which is formed over a substrate having an insulatingsurface. Thin film transistors are applied to a wide range of electronicdevices such as ICs or electro-optical devices, and there is anexpectation for immediate development of thin film transistors to beused, especially, as switching elements for image display devices.Various metal oxides are used for a variety of applications. Indiumoxide is a well-known material and is used as a transparent electrodematerial which is necessary for liquid crystal displays and the like.

Some metal oxides exhibit semiconductor characteristics. Examples ofsuch metal oxides having semiconductor characteristics include tungstenoxide, tin oxide, indium oxide, zinc oxide, and the like. Thin filmtransistors in which a channel formation region is formed of such metaloxides having semiconductor characteristics are known (Patent Documents1 and 2).

REFERENCE Patent Documents

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

In the case where a plurality of thin film transistors is formed over aninsulating surface, there is a portion where a gate wiring and a sourcewiring, for example, intersect each other. At the portion where the gatewiring and the source wiring intersect each other, an insulating layerserving as a dielectric is provided between the gate wiring and thesource wiring whose potential is different from that of the gate wiring,whereby a capacitance is formed. This capacitance can be referred to asa parasitic capacitance between wirings, and may cause distortion of asignal waveform. In addition, when the parasitic capacitance is large,delay in transmission of a signal may occur.

Further, an increase in the parasitic capacitance leads to a cross talkphenomenon in which an electric signal leaks between wirings or anincrease in power consumption.

Furthermore, in an active matrix display device, particularly when alarge parasitic capacitance is formed between a signal wiring supplyinga video signal and another wiring or an electrode, display quality maydeteriorate.

Further, when a circuit is miniaturized, a distance between wirings isreduced and the parasitic capacitance between the wirings may beincreased.

An object of one embodiment of the present invention is to provide asemiconductor device having a structure which enables a sufficientreduction in parasitic capacitance between wirings.

In the case where a plurality of circuits which are different from eachother are formed on an insulating surface, formation of a pixel portionand a drive circuit over one substrate, for example, requires excellentswitching characteristics such as a high on-off ratio of a thin filmtransistor used for the pixel portion. As for a thin film transistorused for the drive circuit, a high operation speed is required. A thinfilm transistor used for the drive circuit preferably operates at a highspeed, since writing time of a display image is reduced, particularly asthe definition of a display device is increased.

Another object is to provide a semiconductor device in which variouscircuits are formed over one substrate and various thin film transistorscorresponding to the required characteristics of the various circuitsare provided while complication of a process and an increase inmanufacturing costs are prevented.

In a bottom-gate thin film transistor, an oxide insulating layer servingas a channel protective layer is formed in contact with part of theoxide semiconductor layer overlapping with a gate electrode layer. Inthe same step as formation of the oxide insulating layer, an oxideinsulating layer covering a peripheral portion (including a sidesurface) of the oxide semiconductor layer is formed.

The oxide insulating layer covering the periphery portion (including theside surface) of the oxide semiconductor layer increases a distancebetween the gate electrode layer and wiring layers (such as a sourcewiring layer and a capacitor wiring layer) formed over or at theperiphery of the gate electrode layer, whereby parasitic capacitance canbe reduced. The oxide insulating layer covering the periphery portion ofthe oxide semiconductor layer is formed in the same formation step asthe channel protective layer. Therefore, parasitic capacitance can bereduced without an increase in the number of fabrication steps.

The oxide insulating layer covering the periphery portion (including theside surface) of the oxide semiconductor layer enables a reduction inparasitic capacitance and suppression of distortion of a signalwaveform.

In order to reduce parasitic capacitance, an insulating material havinga low dielectric constant is preferably used for the oxide insulatinglayer between the wirings.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including: a gate electrode layer over aninsulating surface; a gate insulating layer over the gate electrodelayer, an oxide semiconductor layer over the gate insulating layer, anoxide insulating layer over the oxide semiconductor layer; a sourceelectrode layer and a drain electrode layer over the oxide semiconductorlayer; and an insulating layer over the source electrode layer and thedrain electrode layer.

The oxide semiconductor layer includes a first region in contact withthe oxide insulating layer, a second region in contact with the sourceelectrode layer or the drain electrode layer, and a third region incontact with the insulating layer. In the first region, a channelformation region overlaps with the gate electrode layer with the gateinsulating layer interposed therebetween. The third region is betweenthe channel formation region and the second region.

With the above structure, at least one of the above problems can besolved.

The drain electrode layer formed of a metal electrode of Ti or the likeis in contact with part of an upper surface of the oxide semiconductorlayer, and a high-resistance drain region (also referred to as a HRDregion) which overlaps with the drain electrode layer and is anoxygen-deficient region is formed. Further, the source electrode layeris in contact with part of the upper surface of the oxide semiconductorlayer, and a high-resistance source region (also referred to as a HRSregion) which is an oxygen-deficient region, and overlaps with thesource electrode layer, is formed.

The source electrode layer and the drain electrode layer do not overlapwith the channel formation region in the oxide semiconductor layer.Also, the source electrode layer and the drain electrode layer overlapwith the gate electrode layer with the gate insulating layer interposedtherebetween in an extremely small area or do not overlap with the gateinsulating layer. Accordingly, a reduction in parasitic capacitance isachieved. Furthermore, the width of the oxide insulating layerfunctioning as a channel protection layer is smaller than the distancebetween a side surface of the source electrode layer and a side surfaceof the drain electrode layer which faces the side surface of the sourceelectrode layer. When the oxide insulating layer functioning as thechannel protection layer is designed so as to have a small width (lengthin a channel length direction) for higher speed operation of the thinfilm transistor, the distance between the side surface of the sourceelectrode layer and the side surface of the drain electrode layer whichfaces the side surface of the source electrode layer is also reduced,which may lead to short-circuit the source electrode layer and the drainelectrode layer. Therefore, increasing the distance between the sidesurfaces is effective.

Further, in the above structure, the oxide insulating layer functioningas the channel protection layer is formed by a sputtering method usingan inorganic insulating film, typically a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,or the like.

In the above structure, the oxide semiconductor layer has a region inwhich the upper surface of the oxide semiconductor layer does notoverlap with the oxide insulating layer, nor with the drain electrodelayer or the source electrode layer, i.e., a third region. The width ofthis third region in the channel length direction depends on a positionwhere the oxide semiconductor layer is patterned and a position wherethe drain electrode layer and the source electrode layer are patterned.When the width of this third region in the channel length direction islarge, an off-current of the thin film transistor can be reduced. Incontrast, when the width of this third region in the channel lengthdirection is small, the operation of the thin film transistor can bespeeded up.

The insulating layer in contact with the third region is also formed bya sputtering method using an inorganic insulating film, typically asilicon oxide film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum oxynitride film, or the like. Note that when theinsulating layer in contact with the third region is formed using thesame material as the oxide insulating layer functioning as the channelprotection layer, the oxide insulating layer functioning as the channelprotection layer can be called a first oxide insulating layer and theinsulating layer in contact with the third region can be called a secondoxide insulating layer, rendering the boundary between the first andsecond insulating semiconductors unclear.

As the oxide semiconductor layer, a thin film of InMO₃(ZnO)_(m) (m>0),for example, is formed. The thin film is used as an oxide semiconductorlayer to fabricate a thin film transistor. Note that M denotes one metalelement or a plurality of metal elements selected from Ga, Fe, Ni, Mn,and Co. For example, M may be Ga, or M may contain the above metalelement in addition to Ga:M may be Ga and Ni, Ga and Fe, or the like.Further, in addition to the metal element contained as M, the oxidesemiconductor may contain a transition metal element, such as Fe or Ni,or oxide of the transition metal element as an impurity element. In thisspecification, among the oxide semiconductor layers whose compositionformulas are represented by InMO₃(ZnO)_(m) (m>0), an oxide semiconductorwhose composition formula includes at least Ga as M is referred to as anIn—Ga—Zn—O-based oxide semiconductor, and a thin film of theIn—Ga—Zn—O-based oxide semiconductor is referred to as anIn—Ga—Zn—O-based non-single-crystal film.

As the metal oxide applied to the oxide semiconductor layer, any of thefollowing metal oxide can be applied besides the above: In—Sn—Zn—O-basedmetal oxide; In—Al—Zn—O-based metal oxide; Sn—Ga—Zn—O-based metal oxide;A-Ga—Zn—O-based metal oxide; Sn—Al—Zn—O-based metal oxide; In—Zn—O-basedmetal oxide; Sn—Zn—O-based metal oxide; Al—Zn—O-based metal oxide;In—O-based metal oxide; Sn—O-based metal oxide; and Zn—O-based metaloxide. Alternatively, silicon oxide may be included in the oxidesemiconductor layer formed using the above metal oxide.

The source and drain electrode layers in the above structures are formedusing any of an element selected from Ti, Mo, W, Al, Cr, Cu, and Ta, analloy including any of the elements as a component, an alloy including acombination of any of the elements, and the like. Each of the sourceelectrode layer and the drain electrode layer is not limited to a singlelayer containing the above-described element and may be a stack of twoor more layers.

An embodiment for realizing the above structure is a method of forming asemiconductor device, including the steps of: forming a gate electrodelayer over a substrate having an insulating surface; forming a gateinsulating layer over the gate electrode layer; forming an oxidesemiconductor layer over the gate insulating layer; dehydrating ordehydrogenating the oxide semiconductor layer, wherein the oxidesemiconductor layer is then prevented from being exposed to air and fromfurther water or hydrogen contamination; forming an oxide insulatinglayer being in contact with part of the oxide semiconductor layer andcovering a peripheral portion and a side surface of the oxidesemiconductor layer, forming a source electrode layer and a drainelectrode layer over the oxide insulating layer; and forming aninsulating layer in contact with the oxide insulating layer, the sourceelectrode layer, the drain electrode layer, and the oxide semiconductorlayer.

Dehydration or dehydrogenation is performed by heat treatment in anatmosphere of an inert gas such as nitrogen or a rare gas (such as argonor helium) at a temperature greater than or equal to 400° C. and lessthan the strain point of the substrate, preferably greater than or equalto 420° C. and less than or equal to 570° C., so that the level ofimpurities such as moisture included in the oxide semiconductor layer isreduced.

It can be said that, by heat treatment in an atmosphere of an inert gassuch as nitrogen or a rare gas (e.g., argon or helium), the oxidesemiconductor layer is changed into an oxygen-deficient oxidesemiconductor layer to have lower resistance, i.e., becomes an n-type(n-type) oxide semiconductor layer and that then, by formation of anoxide insulating film in contact with the oxide semiconductor layer andheat treatment after the formation, the oxide semiconductor layer isplaced in an oxygen-excess state to have higher resistance, i.e.,becomes an i-type oxide semiconductor layer. In addition, it can also besaid that solid phase oxidation by which the oxide semiconductor layeris placed in an oxygen-excess state is performed. Thus, it is possibleto manufacture and provide a semiconductor device including a highlyreliable thin film transistor having favorable electric characteristics.

Conditions of the heat treatment are such that when the oxidesemiconductor layer is measured at up to 450° C. by TDS (ThermalDesorption Spectroscopy) after the dehydration or dehydrogenation, twopeaks of water, or at least one peak at around 300° C. is not detected.Therefore, a thin film transistor including oxide semiconductor layerthat has been subjected to the dehydration or dehydrogenation ismeasured at up to 450° C. by TDS, at least the peak of water at around300° C. is not detected.

In addition, when the temperature is lowered from a heat temperature Tat which the dehydration or dehydrogenation is performed, it isimportant to prevent entry of water or hydrogen by using the samefurnace that has been used for the dehydration or dehydrogenation, insuch a manner that the oxide semiconductor layer is not exposed to air.When a thin film transistor is formed using the oxide semiconductorlayer obtained in such a manner that it has lower resistance, i.e.,becomes an n-type (n-type) oxide semiconductor layer by the dehydrationor dehydrogenation and then has higher resistance to become an i-typesemiconductor layer, the threshold voltage of the thin film transistorcan be positive, so that a so-called normally-off switching element canbe realized. It is desirable for a semiconductor device (a displaydevice) that a channel be formed with a threshold voltage having apositive value and as close to 0 V as possible. If the threshold voltageof the thin film transistor is negative, it tends to be normally on; inother words, current flows between the source electrode and the drainelectrode even when the gate voltage is 0 V. In an active-matrix displaydevice, electric characteristics of thin film transistors included in acircuit are important and performance of the display device is dependenton the electric characteristics of thin film transistors. Among theelectric characteristics of thin film transistors, a threshold voltage(Vth) is particularly important. When the threshold voltage value ishigh or is on the minus side even with high field effect mobility, thecircuit is difficult to control. A thin film transistor in which thevalue of the threshold voltage is high and the absolute value of thethreshold voltage is large might not be able to achieve a switchingfunction and might be a load when the transistor is driven at lowvoltage. In the case of an n-channel thin film transistor, it ispreferable that a channel is formed and drain current begins to flowafter the positive voltage is applied as a gate voltage. A transistor inwhich a channel is not formed unless the driving voltage is increasedand a transistor in which a channel is formed and drain current flowseven in the case of the negative voltage state are unsuitable for acircuit.

In addition, a gas atmosphere in which the temperature is lowered fromthe heating temperature T may be switched to a gas atmosphere differentfrom the gas atmosphere in which the temperature is raised to theheating temperature T. For example, cooling is performed by using thesame furnace that is used for the dehydration or dehydrogenation and byfilling the furnace with a high-purity oxygen gas, a high-purity N₂Ogas, or ultra-dry air (having a dew point of −40° C. or less, preferably−60° C. or less) without exposure to air.

With use of oxide semiconductor film obtained in such a manner thatmoisture contained in the film is reduced by the heat treatment for thedehydration and dehydrogenation and slow cooling (or cooling) isperformed in an atmosphere containing no moisture (the dew pointtemperature of which is less than or equal to −40° C. or preferably,less than or equal to −60° C.), electric characteristics of a thin filmtransistor are improved and a thin film transistor having massproductivity and high performance is realized.

In this specification, heat treatment in an inert gas atmosphere ofnitrogen or a rare gas (such as argon or helium) is referred to as heattreatment for dehydration or dehydrogenation. In this specification,dehydrogenation does not refer only to elimination of H₂ by this heattreatment, and the dehydration or dehydrogenation refers to eliminationof H, OH, and the like as well, for the sake of convenience.

By the heat treatment in the atmosphere of an inert gas such as nitrogenor a rare gas (argon, helium, or the like), the oxide semiconductorlayer is changed into an oxygen-deficient oxide semiconductor layer tohave lower resistance, i.e., becomes an n-type (e.g.,n-type) oxidesemiconductor layer.

A region which overlaps with the drain electrode layer and is theoxygen-deficient high-resistance drain (also referred to as HRD) regionis formed.

Further, a region which overlaps with the source electrode layer and isthe oxygen-deficient high-resistance source (also referred to as HRS)region is formed.

Specifically, the carrier concentration of the high-resistance drainregion is greater than or equal to 1×10¹⁸/cm³ and is at least higherthan the carrier concentration of the channel formation region (lessthan 1×10¹⁸/cm³). Note that the carrier concentration in thisspecification is obtained by Hall effect measurement at roomtemperature.

The channel formation region is formed by placing at least part of thedehydrated or dehydrogenated oxide semiconductor layer in anoxygen-excess state to have higher resistance, i.e., to become an i-typeregion. Note that as the treatment for making the dehydrated ordehydrogenated oxide semiconductor layer in an oxygen-excess state,deposition of an oxide insulating film which is in contact with thedehydrated or dehydrogenated oxide semiconductor layer by a sputteringmethod, heat treatment after the deposition of the oxide insulatingfilm, heat treatment after the deposition of the oxide insulating filmin an atmosphere containing oxygen, cooling treatment in an oxygenatmosphere after heat treatment in an inert gas atmosphere after thedeposition of the oxide insulating film, cooling treatment in ultra-dryair (having a dew point of −40° C. or less, preferably −60° C. or less)after heat treatment in an inert gas atmosphere after the deposition ofthe oxide insulating film, or the like is performed.

Further, in order to make part of the dehydrated or dehydrogenated oxidesemiconductor layer (a portion overlapping with a gate electrode layer)serve as the channel formation region, the semiconductor layer can beselectively placed in an oxygen-excess state and have higher resistance,i.e., becomes an i-type semiconductor layer.

Accordingly, it is possible to manufacture and provide a semiconductordevice including a highly reliable thin film transistor having favorableelectric characteristics.

Note that by forming the high-resistance drain region in the oxidesemiconductor layer overlapping with the drain electrode layer, thereliability can be improved when a drive circuit is formed.Specifically, by forming the high-resistance drain region, the drainelectrode layer, the high-resistance drain region, and the channelformation region can vary in conductivity. Therefore, in the case wherethe thin film transistor operates with the drain electrode layerconnected to a wiring for supplying a high power supply potential VDD,the high-resistance region serves as a buffer and a high electric fieldis not applied locally even if the high electric field is appliedbetween the gate electrode layer and the drain electrode layer, whichleads to an increase in the withstand voltage of the transistor.

In addition, by the formation of the high-resistance drain region inpart of the oxide semiconductor layer which overlaps with the drainelectrode layer (and with the source electrode layer), reliability canbe improved when a drive circuit is formed. In particular, by formingthe high-resistance drain region, leakage current between the drainelectrode layer and the source electrode layer of the transistor flowsthrough the drain electrode layer, the high-resistance drain region onthe drain electrode layer side, the channel formation region, thehigh-resistance source region on the source electrode layer side, andthe source electrode layer in this order. In this case, in the channelformation region, leakage current flowing from the high-resistance drainregion on the drain electrode layer side to the channel formation regioncan be concentrated in the vicinity of an interface between the channelformation region and the gate insulating layer which has high resistancewhen the transistor is oft. Thus, the amount of leakage current in aback channel portion (part of a surface of the channel formation whichis apart from the gate electrode layer) can be reduced.

Moreover, as a display device including a drive circuit, alight-emitting display device in which a light-emitting element is usedand a display device in which an electrophoretic display element isused, which is also referred to as an “electronic paper”, are given inaddition to a liquid crystal display device.

In the light-emitting display device using a light-emitting element, aplurality of thin film transistors are included in a pixel portion, andthe pixel portion includes a region where a gate electrode of a thinfilm transistor is connected to a source wiring or a drain wiring ofanother thin film transistor. In addition, a drive circuit of thelight-emitting display device using a light-emitting element includes aregion where a gate electrode of a thin film transistor is connected toa source wiring or a drain wiring of the thin film transistor.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting the thin filmtransistor for the pixel portion is preferably provided over the samesubstrate as a gate line or a source line. The protective circuit ispreferably formed with a non-linear element including an oxidesemiconductor layer.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not indicate the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

It is possible to manufacture a thin film transistor with reducedparasitic capacitance in which the peripheral portion of the oxidesemiconductor layer is covered with the oxide insulating layer.Furthermore, by increasing the distance between the source electrodelayer and the drain electrode layer, a thin film transistor in which anoff-current is reduced can be realized.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views of oneembodiment of the present invention.

FIGS. 2A to 2E are process cross-sectional views of one embodiment ofthe present invention.

FIGS. 3A and 3B are cross-sectional views of one embodiment of thepresent invention.

FIGS. 4A1 to 4B2 are cross-sectional views and top views of oneembodiment of the present invention.

FIGS. 5A and 5B are respectively a cross-sectional view and a top viewof one embodiment of the present invention.

FIGS. 6A and 6B are cross-sectional views of one embodiment of thepresent invention.

FIGS. 7A to 7C are a plan view and cross-sectional views of oneembodiment of the present invention.

FIGS. 8A to 8E are process cross-sectional views of a fabricationprocess of an embodiment of the present invention.

FIGS. 9A and 9B each illustrate a semiconductor device.

FIGS. 10A-1 to 10B each illustrate a semiconductor device.

FIGS. 11A and 11B each illustrate a semiconductor device.

FIG. 12 illustrates a pixel equivalent circuit of a semiconductordevice.

FIGS. 13A to 13C each illustrate a semiconductor device.

FIGS. 14A and 14B are block diagrams each illustrating a semiconductordevice.

FIGS. 15A and 15B are a configuration of a signal line drive circuit anda timing chart illustrating the operation, respectively.

FIGS. 16A to 16D are circuit diagrams of a configuration of a shiftregister.

FIGS. 17A and 17B are respectively a diagram and a timing chartillustrating operation of a shift register.

FIG. 18 illustrates a semiconductor device.

FIG. 19 illustrates a semiconductor device.

FIG. 20 is an external view illustrating an example of an e-book.

FIGS. 21A and 21B are external views respectively illustrating anexample of a television set and an example of a digital photo frame.

FIGS. 22A and 22B are external views each illustrating an example of anamusement machine.

FIGS. 23A and 23B are external views respectively illustrating anexample of a portable computer and an example of a mobile phone.

FIG. 24 illustrates a semiconductor device.

FIG. 25 illustrates a semiconductor device.

FIG. 26 illustrates a semiconductor device.

FIG. 27 illustrates a semiconductor device.

FIG. 28 illustrates a semiconductor device.

FIG. 29 illustrates a semiconductor device.

FIG. 30 illustrates a semiconductor device.

FIG. 31 illustrates a semiconductor device.

FIG. 32 illustrates a semiconductor device.

FIG. 33 illustrates a semiconductor device.

FIG. 34 illustrates a semiconductor device.

FIG. 35 illustrates a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amanufacturing method of the semiconductor device will be described withreference to FIGS. 1A to 1C and FIGS. 2A to 2E.

FIG. 1A is a top view of a channel-protection type thin film transistor448 provided in a pixel, and FIG. 1B is a cross-sectional view takenalong line D1-D2 and D5-D6 of FIG. 1A. FIG. 1C is a cross-sectional viewtaken along line D3-D4. Note that FIG. 2E is the same as FIG. 1B.

The thin film transistor 448 provided in the pixel is achannel-protection (also called channel-stop) type thin film transistor,which includes, over a substrate 400 having an insulating surface, agate electrode layer 421 a, a gate insulating layer 402, an oxidesemiconductor layer 442 including a channel formation region 423, anoxide insulating layer 426 a functioning as a channel protective layer,a source electrode layer 425 a, and a drain electrode layer 425 b. Inaddition, an insulating layer 428 is provided to cover the thin filmtransistor 448 and be in contact with the oxide insulating layer 426 a,the source electrode layer 425 a, and the drain electrode layer 425 b. Aprotective insulating layer 403 and a planarizing insulating layer 404are stacked over the insulating layer 428. Over the planarizinginsulating layer 404, a pixel electrode layer 427 is provided, which isin contact with the drain electrode layer 425 b and thus is electricallyconnected to the thin film transistor 448.

The thin film transistor 448 for a pixel has the oxide semiconductorlayer 442 which includes: a high-resistance source region 424 a whichoverlaps with the source electrode layer; a high-resistance drain region424 b which overlaps with the drain electrode layer; a high-resistancesource region 424 e which does not overlap with the source electrodelayer; a high-resistance drain region 424 f which does not overlap withthe drain electrode layer; and the channel formation region 423. Notethat the high-resistance source region 424 a is formed in contact with alower surface of the source electrode layer 425 a. The high-resistancedrain region 424 b is formed in contact with the lower surface of thedrain electrode layer 425 b. Even when a high electric field is applied,the two high-resistance drain regions or the two high-resistance sourceregions function as buffers, so that the thin film transistor 448 isprevented from being supplied with a high electric field locally and hasan improved withstand voltage.

In FIG. 1B, the channel formation region refers to a region in the oxidesemiconductor layer which is sandwiched, together with the gateinsulating layer, between the oxide insulating layer 426 a and the gateelectrode layer. Therefore, the channel length L of the thin filmtransistor 448 is equal to the width of the oxide insulating layer 426 ain the channel length direction. Note that the channel length L of thethin film transistor 448 means the length of the oxide semiconductorlayer at the interface with the oxide insulating layer 426 a, i.e., thebase of a trapezoid which represents the oxide insulating layer 426 a inthe cross-sectional view of FIG. 1B.

In a wiring intersection portion where a gate wiring and a source wiringintersect each other, the gate insulating layer 402 and an oxideinsulating layer 426 b are provided between the gate electrode layer 421b and the source electrode layer 425 a so that parasitic capacitance canbe reduced. Note that although the oxide insulating layer 426 a whichoverlaps with the channel formation region 423 and the oxide insulatinglayer 426 b which does not overlap with the channel formation region 423are denoted by different reference letters, they are layers formed byusing the same material and formation step.

A process for forming the thin film transistor 448 and the wiringintersection portion over one substrate is described below withreference to FIGS. 2A to 2E. Not only the pixel portion but also a drivecircuit may be formed, and they can be formed over the same substrate bythe same process.

First, after a conductive film is formed over the substrate 400 havingan insulating surface, the gate electrode layers 421 a and 421 b areformed by a first photolithography step. Further, in the pixel portion,a capacitor wiring layer is formed by using the same material and thesame first photolithography step as the gate electrode layers 421 a and421 b. When the drive circuit is formed in addition to the pixel portionand needs to have a capacitor, another capacitor wiring layer is formedin the drive circuit. Note that a resist mask may be formed by an inkjetmethod. When the resist mask is formed by an inkjet method,manufacturing costs can be reduced because a photomask is not used.

As the material of the gate electrode layers 421 a and 421 b, an elementselected from Al, Cr, Ta, Ti, Mo, or W, an alloy including the aboveelement, an alloy including any of these elements in combination, andthe like, can be used. As the conductive film for forming the gateelectrode layers 421 a and 421 b, a light-transmitting conductive filmmay be used. Indium oxide (In₂O₃), an indium oxide-tin oxide alloy(In₂O₃—SnO₂, abbreviated to ITO), or the like can be used.

In this specification, a film transmitting visible light means a filmhaving a thickness such as to have a transmittance of visible light of75% to 100%. In the case where the film has conductivity, the film isreferred to as a transparent conductive film. Further, a conductive filmwhich is semi-transmissive with respect to visible light may be used fora metal oxide applied to the gate electrode layer, the source electrodelayer, the drain electrode layer, the pixel electrode layer, anotherelectrode layer, and the wiring layer. The conductive film which issemi-transmissive with respect to visible light indicates a film havinga transmittance of visible light of 50% to 75%.

In the case where a glass substrate is used and the temperature at whichthe heat treatment to be performed later is high, a glass substratewhose strain point is 730° C. or more is preferably used. As a glasssubstrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. Note that by containing a larger amount of barium oxide (BaO)than that of boric oxide, a glass substrate is heat-resistant and ofmore practical use. Thus, a glass substrate containing BaO and B₂O₃ inwhich the amount of BaO is larger than that of B₂O₃ is preferably used.

Note that, instead of the glass substrate described above, a substrateformed of an insulator such as a ceramic substrate, a quartz substrate,or a sapphire substrate may be used as the glass substrate 400.Alternatively, crystallized glass or the like can be used.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layers 421 a and 421 b. The basefilm has the function of preventing diffusion of an impurity elementfrom the substrate 400, and can have a single-layer or stacked-layerstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film.

Next, the gate insulating layer 402 is formed over the gate electrodelayers 421 a and 421 b.

The gate insulating layer 402 can be a single layer of a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, or a siliconnitride oxide layer, or a stacked layer thereof by a plasma CVD method,a sputtering method, or the like. For instance, a silicon oxynitridelayer may be formed by a plasma CVD method using SiH₄, oxygen, andnitrogen as a film formation gas. The gate insulating layer 402 has athickness from 100 nm to 500 nm inclusive. In the case of a stackedstructure, a first gate insulating layer having a thickness from 50 nmto 200 nm inclusive and a second gate insulating layer having athickness from 5 nm to 300 nm inclusive are stacked in this order.

In this embodiment, the gate insulating layer 402 is a silicon nitridelayer having a thickness of 200 nm or less and formed by a plasma CVDmethod.

Next, over the gate insulating layer 402, an oxide semiconductor film430 is formed to a thickness greater than or equal to 5 nm and less thanor equal to 200 nm, preferably greater than or equal to 10 nm and lessthan or equal to 20 nm (see FIG. 2A). The oxide semiconductor film 430preferably has a thickness as small as 50 nm or less so as to beamorphous even when the heat treatment for dehydration ordehydrogenation follows the formation of the oxide semiconductor film430. By reducing the thickness of the oxide semiconductor film,crystallization thereof can be suppressed when the heat treatmentfollows the formation of the oxide semiconductor layer.

As the oxide semiconductor film 430, an In—Ga—Zn—O-basednon-single-crystal film, an In—Sn—Zn—O-based oxide semiconductor film,an In—Al—Zn—O-based oxide semiconductor film, a Sn—Ga—Zn—O-based oxidesemiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film, aSn—Al—Zn—O-based oxide semiconductor film, an In—Zn—O-based oxidesemiconductor film, a Sn—Z—O-based oxide semiconductor film, anAl—Zn—O-based oxide semiconductor film, an In—O-based oxidesemiconductor film, a Sn—O-based oxide semiconductor film, or aZn—O-based oxide semiconductor film is used. In this embodiment, theoxide semiconductor film 430 is formed using an In—Ga—Zn—O-based oxidesemiconductor target by a sputtering method. Alternatively, the oxidesemiconductor film 430 can be formed by a sputtering method in a raregas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphereof a rare gas (typically argon) and oxygen. When a sputtering method isemployed, it is preferable that deposition be performed using a targetcontaining a 2 to 10 wt % SiO₂ and that SiOx (x>0) which inhibitscrystallization be contained in the oxide semiconductor film 430 so asto prevent crystallization at the time of the heat treatment fordehydration or dehydrogenation in a later step.

The oxide semiconductor layer is preferably an oxide semiconductorcontaining In, more preferably an oxide semiconductor containing In andGa. In order to obtain an i-type (intrinsic) oxide semiconductor,subjecting the oxide semiconductor to dehydration or dehydrogenation iseffective.

In this embodiment, an In—Ga—Zn—O-based oxide semiconductor film isused.

Here, the oxide semiconductor film is formed using an oxidesemiconductor target containing In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1(molar ratioj) under conditions where the distance between the substrateand the target is 100 mm, the pressure is 0.2 Pa, and the direct current(DC) power supply is 0.5 kW in an atmosphere containing argon and oxygen(argon:oxygen=30 sccm:20 sccm, the proportion of the oxygen flow is40%). Note that use of a pulse direct current (DC) power supply ispreferable because dust can be reduced and the film thickness can beuniform in this case. The In—Ga—Zn—O-based non-single-crystal film isformed to a thickness of 5 nm to 200 nm. In this embodiment, as theoxide semiconductor film, an In—Ga—Zn—O-based non-single-crystal filmhaving a thickness of 20 nm is formed using the In—Ga—Zn—O-based oxidesemiconductor target by a sputtering method.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. An RF sputtering method is mainly used toform an insulating film, while a DC sputtering method is mainly used toform a metal film.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed and stacked in the same chamber, or a film can be formed ofplural kinds of materials by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method by a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin compound film thereof, and a bias sputteringin which a voltage is also applied to a substrate during deposition.

Then, the oxide semiconductor film 430 is processed into island-shapedoxide semiconductor layers by a second photolithography step. The resistmask for forming the island-shaped semiconductor layers may be formed byan inkjet method. Formation of the resist mask by an inkjet method doesnot require a photomask, which results in a reduction in manufacturingcosts.

Next, the oxide semiconductor layer is subjected to dehydration ordehydrogenation. First heat treatment for dehydration or dehydrogenationis performed at a temperature greater than or equal to 400° C., and lessthan the strain point of the substrate, preferably 425° C. or more. Notethat in the case where the temperature is 425° C. or more, the heattreatment time may be one hour or less, whereas in the case where thetemperature is less than 425° C., the heat treatment time is longer thanone hour. Here, the substrate is introduced into an electric furnacewhich is one of heat treatment apparatuses, and the heat treatment ofthe oxide semiconductor layer is performed in a nitrogen atmosphere.After that, the oxide semiconductor layer is not exposed to air, whichprevents the oxide semiconductor layer from further water or hydrogencontamination; thus, an oxide semiconductor layer is obtained. In thisembodiment, one furnace employed in the heat treatment is continuouslyused up to the time when the temperature is lowered from the heattemperature Tat which dehydration or dehydrogenation of the oxidesemiconductor layer is performed to a temperature low enough to preventreincorporation of water. Specifically, slow cooling is performed in anitrogen atmosphere up to the time when the temperature becomes lessthan the heat temperature T by 100° C. or more. Without being limited toa nitrogen atmosphere, the dehydration or dehydrogenation is performedin a rare gas atmosphere, such as helium, neon, or argon.

Note that in the first heat treatment, it is preferable that moisture,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. In addition, nitrogen or a rare gas such ashelium, neon, or argon which is introduced into the heat treatmentapparatus preferably has a purity of 6N (99.9999%) or more, morepreferably 7N (99.99999%) or more (i.e., the concentration of impuritiesis 1 ppm or less, preferably 0.1 ppm or less).

Further, the oxide semiconductor film can be crystallized to be a microcrystal film or a polycrystalline film depending on a condition of thefirst heat treatment or a material of oxide semiconductor layer.

Alternatively, the first heat treatment for the oxide semiconductorlayer can be performed with respect to the oxide semiconductor film 430before it is processed into the island-shaped oxide semiconductorlayers. In that case, after the first heat treatment, the substrate istaken out of the heating apparatus and a photolithography step isperformed.

Furthermore, heat treatment (heating temperature is 400° C. or more andless than the distortion point of the substrate) may be performed in aninert gas atmosphere (e.g., nitrogen, helium, neon, or argon). an oxygenatmosphere, or reduced pressure before deposition of the oxidesemiconductor film 430, so that impurities such as hydrogen and water inthe gate insulating layer are removed.

Next, an oxide insulating film is formed over the gate insulating layer402 and the oxide semiconductor layer by a sputtering method. Then, aresist mask is formed by a third photolithography step, and the oxideinsulating layers 426 a and 426 b are formed by selective etching. Afterthat, the resist mask is removed. At this stage, in the oxidesemiconductor layer, there are regions in contact with the oxideinsulating layers. Among these regions, the region that overlaps withthe gate electrode layer with the gate insulating layer interposedtherebetween and also overlaps with the oxide insulating layer 426 a isthe channel formation region. There is also the region that overlapswith the oxide insulating layer 426 b covering the peripheral portionand side surface of the oxide semiconductor layer.

The oxide insulating film can be formed to a thickness at least 1 nm ormore by a sputtering method, or the like, as appropriate, which is amethod with which impurities such as water and hydrogen are not mixedinto the oxide insulating film. In this embodiment, a 300-nm-thicksilicon oxide film is formed by a sputtering method, as the oxideinsulating film. The substrate temperature in film formation may begreater than or equal to room temperature and less than or equal to 300°C. In this embodiment, the substrate is at room temperature. Theformation of the silicon oxide film by a sputtering method can beperformed in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or an atmosphere of a rare gas (typically, argon) andoxygen. As a target, a silicon oxide target or a silicon target can beused. For example, with use of a silicon target, a silicon oxide filmcan be formed by a sputtering method in an atmosphere of oxygen andnitrogen. As the oxide insulating film formed in contact with the oxidesemiconductor layer whose resistance is reduced, an inorganic insulatingfilm that does not include impurities such as moisture, a hydrogen ion,and OH⁻ and blocks entry of these from the outside is used.Specifically, a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, an aluminum oxynitride film, or the like is used.

Next, second heat treatment is performed in an inert-gas atmosphere or anitrogen atmosphere, preferably at a temperature of 200° C. to 400° C.inclusive, e.g., of 250° C. to 350° C. inclusive (see FIG. 2B). Forexample, second heat treatment is performed at 250° C. for one hour in anitrogen atmosphere. In the second heat treatment, heat is applied whilethe oxide insulating layer is in contact with end portions of the oxidesemiconductor layer 442 which overlap with the oxide insulating layer426 b and with part of the region in the oxide semiconductor layer 442which overlaps with the oxide insulating layer 426 a. Note that in thesecond heat treatment, the oxide semiconductor layer 442 is heated witha portion that does not overlap with the oxide insulating layer beingexposed. When heat treatment is performed in a nitrogen atmosphere or aninert-gas atmosphere where the oxide semiconductor layer 442 is exposed,resistance can be reduced in the high-resistance (i-type) region whichis exposed in the oxide semiconductor layer 442. The oxide insulatinglayer 426 a is provided above and in contact with the channel formationregion of the oxide semiconductor layer 442, and functions as a channelprotective layer.

Next, a conductive film is formed over the gate insulating layer 402,the oxide insulating layers 426 a and 426 b, and the oxide semiconductorlayer 442. After that, through a fourth photolithography step, a resistmask is formed, and etching is performed selectively to form the sourceelectrode layer 425 a and the drain electrode layer 425 b (see FIG. 2C).The conductive film is formed by a sputtering method, a vacuumevaporation method (e.g., an electron beam evaporation method, or thelike), an arc discharge ion plating method, or a spray method. As thematerial of the conductive film, an element selected from Al, Cr, Cu,Ta, Ti, Mo, and W, an alloy containing any of these elements as acomponent, an alloy containing these elements in combination, and thelike can be used. The conductive film is not limited to a single layercontaining the above-described element and may be a stack of two or morelayers. In this embodiment, the conductive film has a three-layerstructure of a titanium film, an aluminum film, and a titanium film.Instead of a Ti film, a titanium nitride film may be used.

In the fourth photolithography step, only portions of the conductivefilm which are above and in contact with the oxide semiconductor layerare removed. By using an ammonia peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2), or the like as an alkaline etchant sothat only the portions of the conductive film which are above and incontact with the oxide semiconductor layer are thus removed, it ispossible to selectively remove the conductive film and to leave theoxide semiconductor layer including an In—Ga—Zn—O-based oxidesemiconductor.

The resist mask for forming the source electrode layer 425 a and thedrain electrode layer 425 b may be formed by an inkjet method. Formationof the resist mask by an inkjet method does not require a photomask,which results in a reduction in manufacturing costs.

Next, the insulating layer 428 and the protective insulating layer 403are formed over the oxide insulating layers 426 a and 426 b, the sourceelectrode layer 425 a, and the drain electrode layer 425 b. In thisembodiment, the insulating layer 428 made of a silicon oxide film andthe protective insulating layer 403 made of a silicon nitride film areformed by using a sputtering method and stacked.

Note that the boundary between the oxide insulating layer 426 a and theinsulating layer 428 is illustrated in the drawings for betterunderstanding, but is actually unclear because these layers are bothsilicon oxide films formed by a sputtering method.

Since an RF sputtering method allows high productivity, it is preferablyused for deposition of the protective insulating layer 403. Theprotective insulating layer 403 is formed using an inorganic insulatingfilm that does not contain impurities such as moisture, hydrogen ions,and OH⁻ and blocks entry of such impurities from the outside, typicallya silicon nitride film, an aluminum nitride film, a silicon nitrideoxide film, or an aluminum oxynitride film. Needless to say, theprotective insulating layer 403 is a light-transmitting insulating film.

Then, the planarizing insulating layer 404 is formed over the protectiveinsulating layer 403. The planarizing insulating layer 404 can be formedfrom an organic material having heat resistance, such as polyimide,acrylic resin, benzocyclobutene-based resin, polyamide, or epoxy resin.Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. The planarizing insulating layer 404 may be formed by stackingplural insulating films formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

The formation method of the planarizing insulating layer 404 is notlimited to a particular method, and the following method can be useddepending on the material: a sputtering method, an SOG method, a spincoating method, a dipping method, a spray coating method, a dropletdischarge method (e.g., an inkjet method, screen printing, offsetprinting, or the like), and the like. Further, the planarizinginsulating layer 404 can be formed with a doctor knife, a roll coater, acurtain coater, a knife coater, or the like.

Next, by performing a fifth photolithography step, a resist mask isformed, and the planarizing insulating layer 404, the insulating layer428 and the protective insulating layer 403 are etched, so that acontact hole 441 which reaches the drain electrode layer 425 b isformed. The resist mask is then removed (see FIG. 2D). As illustrated inFIG. 2D, the oxide insulating layer 426 b is provided below the contacthole, which results in that the thickness of the planarizing insulatinglayer to be removed can be small as compared to the case where the oxideinsulating layer is not provided below the contact hole; accordingly,etching time can be shortened. In addition, as compared to the casewhere the oxide insulating layer is not provided below the contact hole,the depth of the contact hole 441 can be shallow; accordingly, thecoverage with a light-transmitting conductive film, which is formedlater and located in a region overlapping with the contact hole 441, canbe eased. Besides, a contact hole reaching the gate electrode layer 421b is also formed in this etching. A resist mask used for formation ofthe contact hole reaching the drain electrode layer 425 b may be formedby an inkjet method. Formation of the resist mask by an inkjet methodrequires no photomask; thus, manufacturing costs can be reduced.

Next, a light-transmitting conductive film is formed. Thelight-transmitting conductive film can be formed using indium oxide(In₂O₃), an alloy of indium oxide and tin oxide (In₂O₃—SnO₂, abbreviatedto ITO), or the like by a sputtering method, a vacuum evaporationmethod, or the like. Alternatively, an Al—Zn—O-based non-single-crystalfilm containing nitrogen (i.e., an Al—Zn—O—N-based non-single-crystalfilm), a Zn—O—N-based non-single-crystal film, or a Sn—Zn—O—N basednon-single-crystal film may be used. Note that the percentage (atomic %)of zinc in the Al—Zn—O—N-based non-single-crystal film is 47 atomic % orless and is higher than that of aluminum in the non-single-crystal film;the percentage (atomic %) of aluminum in the non-single-crystal film ishigher than that of nitrogen in the non-single-crystal film. Such amaterial is etched with a hydrochloric acid-based solution. However,since a residue is easily generated, particularly in ITO etching, analloy of indium oxide and zinc oxide (In₂O₃—ZnO) may be used to improveetching processability.

Note that the unit of the percentage of components in thelight-transmitting conductive film is atomic percent, and the percentageof components is evaluated by analysis using an electron probe X-raymicroanalyzer (EPMA).

Next, by performing a sixth photolithography step, a resist mask isformed, an unnecessary portion is removed by etching to form a pixelelectrode layer 427, and then, the resist mask is removed (see FIG. 2E).

Through the above steps, with use of the six masks, the thin filmtransistor 448 and the wiring intersection portion with reducedparasitic capacitance can be formed over one substrate. The thin filmtransistor 448 for a pixel is a channel-protective thin film transistorhaving the oxide semiconductor layer 442 which includes: thehigh-resistance source region 424 a which overlaps with the sourceelectrode layer; the high-resistance drain region 424 b which overlapswith the drain electrode layer; the high-resistance source region 424 ewhich does not overlap with the source electrode layer; thehigh-resistance drain region 424 f which does not overlap with the drainelectrode layer, and the channel formation region 423. Thus, even when ahigh electric field is applied, the two high-resistance drain regions orthe two high-resistance source regions function as buffers, so that thethin film transistor 448 is prevented from being supplied with a highelectric field locally and has an improved withstand voltage.Furthermore, by increasing the distance between the source electrodelayer and the drain electrode layer, an off-current of the thin filmtransistor is reduced.

In addition, a storage capacitor formed with the capacitor wiring layerand the capacitor electrode, in which the gate insulating layer 402 isused as a dielectric, can be formed over the same substrate as the thinfilm transistor 448. By arranging the thin film transistor 448 and thestorage capacitor in each pixel of a pixel portion in which pixels arearranged in a matrix form, one of the substrates for manufacturing anactive matrix display device can be obtained. In this specification,such a substrate is referred to as an active matrix substrate forconvenience.

Further, a thin film transistor for a drive circuit can be provided overthe same substrate as the pixel portion. By providing the drive circuitand the pixel portion over one substrate, connection wirings between thedrive circuit and an external signal can be shortened; thus, the sizeand costs of the semiconductor device can be reduced.

In the thin film transistor 448 for a pixel which is illustrated in FIG.1B, the oxide semiconductor layer 442 is provided with a first region424 c and a second region 424 d which overlap with the oxide insulatinglayer 426 b and are positioned in the peripheral portion of the oxidesemiconductor layer 442. The first region 424 c and the second region424 d which are peripheral portions of the oxide semiconductor layer 442are in an oxygen-excess state, as well as the channel formation region423. By the first region 424 c and the second region 424 d, a leakcurrent and parasitic capacitance can be reduced when the oxidesemiconductor layer and a wiring having a potential different from thepotential of the oxide semiconductor layer are arranged to be closedeach other.

In terms of high integration, it is preferable that especially in thedrive circuit, a plurality of wirings and a plurality of oxidesemiconductor layers be arranged at small distances therebetween. Insuch a drive circuit, the first region 424 c and the second region 424 dare provided by overlapping the oxide semiconductor layer with the oxideinsulating layer 426 b, which is effective to reduce a leak current andparasitic capacitance. Further, in the case where a plurality of thinfilm transistors are arranged in series or in parallel, an oxidesemiconductor layer is formed in one island among the plurality of thinfilm transistors, and the oxide semiconductor layer overlaps with theoxide insulating layer 426 b, whereby isolation element is formed. Thus,a region overlapping with the oxide insulating layer 426 b is anisolation element region. In such a manner, a plurality of thin filmtransistors can be arranged in a small area, so that the drive circuitcan be highly integrated.

Embodiment 2

In this embodiment is described an example in which an active matrixliquid crystal display device is manufactured by using the thin filmtransistor described in Embodiment 1 to form a pixel portion and a drivecircuit over one substrate.

FIG. 3A illustrates an example of a cross-sectional structure of theactive matrix substrate.

Although only the thin film transistor in the pixel portion and thewiring intersection portion are shown in Embodiment 1, the thin filmtransistor in the drive circuit, the storage capacitor, the gate wiring,and a terminal portion of the source wiring are shown in thisembodiment, as well as the thin film transistor and the wiringintersection portion. The capacitor, the gate wiring, and the terminalportion of the source wiring can be formed by the same process as themanufacturing process described in Embodiment 1.

In FIG. 3A, a thin film transistor 220 electrically connected to a pixelelectrode layer 227 is of a channel-protection type and is provided inthe pixel portion. In this embodiment, the transistor having the samestructure as the thin film transistor 448 of Embodiment 1 is used.

A capacitor wiring layer 230, which is formed using the samelight-transmitting conductive material and formation step as the gateelectrode layer of the thin film transistor 220, overlaps with acapacitor electrode 231, with a gate insulating layer 202 serving as adielectric interposed therebetween, thereby forming the storagecapacitor. Note that the capacitor electrode 231 is formed using thesame metal material and formation step as the source electrode layer anddrain electrode layer of the thin film transistor 220.

Note that the storage capacitor is provided below the pixel electrodelayer 227, and the capacitor electrode 231 is electrically connected tothe pixel electrode layer 227.

An example in which the storage capacitor is formed using the capacitorelectrode 231 and the capacitor wiring layer 230 is described in thisembodiment, but there is no particular limitation on the structureforming the storage capacitor. For instance, the storage capacitor maybe formed in such a manner that the pixel electrode layer overlaps withthe gate wiring of an adjacent pixel with the planarizing insulatinglayer, the protective insulating layer, and the gate insulating layerinterposed therebetween, without the provision of the capacitor wiringlayer.

In FIG. 3A, the storage capacitor includes only the gate insulatinglayer 202 between the capacitor wiring layer and the capacitor electrodeso that a large capacitance is formed, while the wiring intersectionportion includes the gate insulating layer 202 and an oxide insulatinglayer 266 b which are provided between the gate electrode layer 421 band a wiring formed above the gate electrode layer 421 b so thatparasitic capacitance is reduced. In the storage capacitor, in orderthat only the gate insulating layer 202 be provided between thecapacitor wiring layer and the capacitor electrode, an etching conditionor a material of the gate insulating layer is selected such that etchingfor removing the oxide insulating layer 266 b allows only the gateinsulating layer 202 to remain. In this embodiment, because the oxideinsulating layer 266 b is a silicon oxide film formed by a sputteringmethod and the gate insulating layer 202 is a silicon nitride filmformed by a plasma CVD method, the selective removal can be realized.Note that when a material that causes the oxide insulating layer 266 band the gate insulating layer 202 to be removed under the sameconditions is used, the thicknesses are preferably set such that atleast part of the gate insulating layer can remain and form thecapacitor even if the thickness of the gate insulating layer is partlyreduced. Since the thickness of the gate insulating layer is preferablyreduced in order to form large capacitance in the storage capacitor, thethickness of part of the gate insulating layer above the capacitorwiring may be reduced during the etching for removing the oxideinsulating layer 266 b.

A thin film transistor 260 is of a channel-protection type and isprovided in the drive circuit. The thin film transistor 260 has ashorter channel length L than the thin film transistor 220 so thathigher operation speed is realized. The channel length L of the thinfilm transistor of a channel-protection type which is provided in thedrive circuit is preferably set to greater than or equal to 0.1 μm andless than or equal to 2 μm. The thin film transistor 260 has a structurein which a source electrode layer 26 a and a drain electrode layer 265 bare formed so as to overlap with the oxide insulating layer 266 a, whichis different from the structure of the thin film transistor 220.

The thin film transistor 260 includes: over a substrate 200 having aninsulating surface, a gate electrode layer 261; the gate insulatinglayer 202; the oxide semiconductor layer having at least a channelformation region 263, a high-resistance source region 264 a, and ahigh-resistance drain region 264 b; the source electrode layer 265 a;and the drain electrode layer 265 b. Further, the oxide insulating layer266 a in contact with the channel formation region 263 is also provided.

The gate electrode layer of the thin film transistor 260 in the drivecircuit may be electrically connected to a conductive layer 267 providedabove the oxide semiconductor layer. In that case, a planarizinginsulating layer 204, an insulating layer 216, a protective insulatinglayer 203, the oxide insulating layer 266 b, and the gate insulatinglayer 202 are selectively etched using the same photomask as the contacthole for electrically connecting the drain electrode layer of the thinfilm transistor 220 to the pixel electrode layer 227, whereby a contacthole is formed. Through this contact hole, the conductive layer 267 iselectrically connected to the gate electrode layer 261 of the thin filmtransistor 260 in the drive circuit.

For the insulating layer 216, an inorganic insulating film such as asilicon oxide film, an aluminum oxide film, a silicon oxynitride film,or an aluminum oxynitride film is used. In this embodiment, a siliconoxide film formed by a sputtering method is used.

For the protective insulating layer 203, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum oxynitride film is used. In thisembodiment, a silicon nitride film formed by a sputtering method isused.

In the thin film transistor 260, the width of the gate electrode layer261 (the width in the channel length direction) is larger than that ofthe oxide semiconductor layer. The oxide insulating layer 266 b overlapswith a peripheral portion of the oxide semiconductor layer and alsooverlaps with the gate electrode layer 261. The oxide insulating layer266 b is used to increase the distance between the drain electrode layer265 b and the gate electrode layer 261, and to reduce the parasiticcapacitance formed between the drain electrode layer 265 b and the gateelectrode layer 261. A first region 264 c and a second region 264 d inthe oxide semiconductor layer which overlap with the oxide insulatinglayer 266 b are in an oxygen-excess state like the channel formationregion 263, and reduce leakage current and also reduce the parasiticcapacitance.

Plural gate wirings, source wirings, and capacitor wiring layers areprovided in accordance with the pixel density. In the terminal portion,plural first terminal electrodes at the same potential as the gatewiring, plural second terminal electrodes at the same potential as thesource wiring, plural third terminal electrodes at the same potential asthe capacitor wiring layer, and the like are arranged. The number ofeach of the terminal electrodes may be any number determined asappropriate by the practitioner.

In the terminal portion, the first terminal electrode at the samepotential as the gate wiring can be formed using the samelight-transmitting material as the pixel electrode layer 227. The firstterminal electrode is electrically connected to the gate wiring througha contact hole reaching the gate wiring. The contact hole reaching gatewiring is formed in such a manner that the planarizing insulating layer204, the insulating layer 216, the protective insulating layer 203, theoxide insulating layer 266 b, and the gate insulating layer 202 areselectively etched using the same photomask as the contact hole forelectrically connecting the drain electrode layer of the thin filmtransistor 220 to the pixel electrode layer 227.

The second terminal electrode 255 at the same potential as the sourcewiring 254 in the terminal portion can be formed using the samelight-transmitting material as the pixel electrode layer 227. The secondterminal electrode 255 is electrically connected to the source wiring254 through a contact hole reaching the source wiring 254. The sourcewiring is a metal wiring formed using the same material and step as thesource electrode layer 265 a of the thin film transistor 260, and is atthe same potential as the source electrode layer 265 a.

The third terminal electrode at the same potential as the capacitorwiring layer 230 can be formed using the same light-transmittingmaterial as the pixel electrode layer 227. A contact hole reaching thecapacitor wiring layer 230 can be formed by using the same photomask andstep as a contact hole for electrically connecting the capacitorelectrode 231 to the pixel electrode layer 227.

When an active matrix liquid crystal display device is manufactured, anactive matrix substrate and a counter substrate provided with a counterelectrode are fixed with a liquid crystal layer therebetween. Note thata common electrode electrically connected to the counter electrodeprovided for the counter substrate is provided over the active matrixsubstrate, and the fourth terminal electrode electrically connected tothe common electrode is provided in the terminal portion. This fourthterminal electrode is a terminal for setting the common electrode at afixed potential such as GND or 0 V. The fourth terminal electrode can beformed using the same light-transmitting material as the pixel electrodelayer 227.

When the gate electrode layer, the source electrode layer, the drainelectrode layer, the pixel electrode layer, another electrode layer, andanother wiring layer are formed using the same material, a commonsputtering target or a common manufacturing apparatus can be used.Accordingly, the costs of the material of these layers and an etchant(or an etching gas) used in etching can be reduced, resulting in areduction in manufacturing costs.

In the structure of FIG. 3A, when a photosensitive resin material isused for the planarizing insulating layer 204, the step of forming aresist mask can be omitted.

FIG. 3B illustrates a cross-sectional structure that is partly differentfrom that illustrated in FIG. 3A. FIG. 3B is the same as FIG. 3A exceptthat there is no planarizing insulating layer 204 in the terminalportion and except for the structure of the thin film transistor in thedrive circuit. Therefore, components common to FIGS. 3A and 3B keep thesame reference numerals, and are not described in detail again. In FIG.3B, a thin film transistor 270 using a metal wiring is provided. Theterminal electrode is also formed by using the same material and processas the metal wiring.

In the structure of FIG. 3B, a photosensitive resin material is used forthe planarizing insulating layer 204, and the step of forming a resistmask is omitted. Thus, without use of a resist mask, a structure inwhich there is no planarizing insulating layer 204 in the terminalportion can be formed. When there is no planarizing insulating layer inthe terminal portion, good connection to an FPC (Flexible PrintedCircuit) can easily be obtained.

The thin film transistor 270 includes: over the substrate 200 having aninsulating surface, a gate electrode layer 271; the gate insulatinglayer 202; the oxide semiconductor layer having at least a channelformation region 273, a high-resistance source region 274 a, and ahigh-resistance drain region 274 b; a source electrode layer 275 a; anda drain electrode layer 275 b. Further, an oxide insulating layer 276 ain contact with the channel formation region 273 is also provided.Furthermore, the insulating layer 216 and the protective insulatinglayer 203 are formed over the source electrode layer 275 a and the drainelectrode layer 275 b.

A first region 274 c and a second region 274 d in the oxidesemiconductor layer which overlap with the oxide insulating layer 276 bare in an oxygen-excess state like the channel formation region 273, andserve to reduce leakage current and also reduce the parasiticcapacitance. A third region 274 e in the oxide semiconductor layer whichis in contact with the insulating layer 216 is provided between thechannel formation region 273 and the high-resistance source region 274a. A fourth region 274 f in the oxide semiconductor layer which is incontact with the insulating layer 216 is provided between the channelformation region 273 and the high-resistance drain region 274 b. Thethird region 274 e and the fourth region 274 f in the oxidesemiconductor layer which are in contact with the insulating layer 216enables a reduction in off-current.

In the thin film transistor of a channel-protection type, when thesource electrode layer and the drain electrode layer are provided overthe oxide insulator layer having a small width which is reduced so as toshorten the channel length L of the channel formation region, a shortcircuit could be formed over the oxide insulating layer. Therefore, thesource electrode layer 275 a and the drain electrode layer 275 b areprovided so that their end portions are apart from the oxide insulatinglayer 276 a having a small width.

The gate electrode layer of the thin film transistor 270 in the drivecircuit may be electrically connected to a conductive layer 277 providedabove the oxide semiconductor layer.

The second terminal electrode 257 at the same potential as the sourcewiring 256 in the terminal portion can be formed of the samelight-transmitting material as the pixel electrode layer 227. The sourcewiring is a metal wiring formed using the same material and step as thesource electrode layer 275 a of the thin film transistor 270, and is atthe same potential as the source electrode layer 275 a.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit is preferably provided over the samesubstrate as the pixel portion or the drive circuit. The protectivecircuit is preferably formed using a non-linear element including anoxide semiconductor layer. For example, protective circuits are providedbetween the pixel portion and a scan line input terminal and between thepixel portion and a signal line input terminal. In this embodiment, aplurality of protective circuits are provided so as to prevent breakageof the a pixel transistor and the like which can be caused when a surgevoltage due to static electricity or the like is applied to a scan line,a signal line, and a capacitor bus line. Therefore, the protectivecircuit is formed so as to release charge to a common wiring when asurge voltage is applied to the protective circuit. Further, theprotective circuit includes non-linear elements arranged in parallel toeach other with the scan line therebetween. The non-linear elementincludes a two-terminal element such as a diode or a three-terminalelement such as a transistor. For example, the non-linear element canalso be formed through the same step as the thin film transistor 220 inthe pixel portion, and can be made to have the same properties as adiode by connecting a gate terminal to a drain terminal of thenon-linear element.

A structure in which the planarizing insulating layer 204 is notprovided may be employed by omitting the step of forming the planarizinginsulating layer 204. In this case, the pixel electrode layer 227 andthe second terminal electrode 255 are provided above and in contact withthe protective insulating layer 203.

This embodiment can be combined with Embodiment 1 as appropriate.

Embodiment 3

In this embodiment, an example of a structure of a terminal portionprovided over the same substrate as the thin film transistor isdescribed. Although an example of the terminal portion of the sourcewiring is described in Embodiment 2, a terminal portion of the sourcewiring which is different from the terminal portion described inEmbodiment 2 and a terminal portion of the gate wiring are shown in thisembodiment. Note that in FIGS. 4A1 to 4B2, components common to FIGS. 3Aand 3B keep the same reference numerals.

FIGS. 4A1 and 4A2 respectively illustrate a cross-sectional view and atop view of the terminal portion of the gate wiring. FIG. 4A1 is thecross-sectional view taken along line C1-C2 of FIG. 4A2. In FIG. 4A1, aconductive layer 225 formed over a stack of the insulating layer 216 andthe protective insulating layer 203 is a terminal electrode forconnection which functions as an input terminal. Furthermore, in theterminal portion of FIG. 4A, a first terminal 221 formed of the samematerial as the gate electrode layer 421 b of FIG. 2E and a connectionelectrode layer 223 formed of the same material as the source wiringoverlap with each other with the gate insulating layer 202 interposedtherebetween, and are electrically connected to each other through theconductive layer 225.

FIGS. 4B1 and 4B2 are respectively a cross-sectional view and a top viewof the terminal portion of the source wiring which is different from thesource wiring terminal portion illustrated in FIG. 3B. FIG. 4B1 is thecross-sectional view taken along line C3-C4 of FIG. 4B2. In FIG. 4B1, aconductive layer 225 formed over a stack of the insulating layer 216 andthe protective insulating layer 203 is the terminal electrode forconnection which functions as an input terminal. Furthermore, in theterminal portion of FIG. 4B1, an electrode layer 226 formed of the samematerial as the gate wiring is located below and overlaps with a secondterminal 222, which is electrically connected to the source wiring, withthe gate insulating layer 202 interposed therebetween. The electrodelayer 226 is not electrically connected to the second terminal 222. Whenthe potential of the electrode layer 226 is set to a potential differentfrom that of the second terminal 222, such as floating, GND, or 0 V, acapacitor for preventing noise or static electricity can be formed. Inaddition, the second terminal 222 is electrically connected to theconductive layer 225 through a contact hole formed in the stack of theinsulating layer 216 and the protective insulating layer 203.

Plural gate wirings, source wirings, and capacitor wirings are providedin accordance with the pixel density. In the terminal portion, pluralfirst terminals at the same potential as the gate wiring, plural secondterminals at the same potential as the source wiring, plural thirdterminals at the same potential as the capacitor wiring, and the likeare arranged. The number of each of the terminal electrodes may be anynumber determined as appropriate by the practitioner.

This embodiment can be combined with Embodiment 1 or 2 as appropriate.

Embodiment 4

In this embodiment, a description is provided of an example of a liquidcrystal display device in which a liquid crystal layer is sealed betweena first substrate and a second substrate, and a common connectionportion electrically connected to the counter electrode provided for thesecond substrate is formed over the first substrate. Note that a thinfilm transistor is formed as a switching element over the firstsubstrate, and the common connection portion is manufactured in the sameprocess as the switching element in the pixel portion, thereby beingobtained without complicating the process.

The common connection portion is provided in a position that overlapswith a sealant for bonding the first substrate and the second substrate,and is electrically connected to the counter electrode throughconductive particles contained in the sealant. Alternatively, the commonconnection portion is provided in a position that does not overlap withthe sealant (except for the pixel portion) and a paste containingconductive particles is provided separately from the sealant so as tooverlap with the common connection portion, whereby the commonconnection portion is electrically connected to the counter electrode.

FIG. 5A is a cross-sectional view of a semiconductor device in which athin film transistor and a common connection portion are formed over onesubstrate.

In FIG. 5A, the thin film transistor 220 electrically connected to thepixel electrode layer 227 is of a channel-protection type and isprovided in the pixel portion. In this embodiment, the thin filmtransistor 220 has the same structure as the thin film transistor 448 ofEmbodiment 1.

FIG. 5B illustrates an example of a top view of the common connectionportion, and dashed line C5-C6 in FIG. 5B corresponds to a cross sectionof the common connection portion of FIG. 5A. Note that in FIG. 5B,components common to FIG. 5A keep the same reference numerals.

A common potential line 205 is provided over the gate insulating layer202, and formed by using the same material and step as the sourceelectrode layer and the drain electrode layer of the thin filmtransistor 220.

Also, the common potential line 205 is covered with the stack of theinsulating layer 216 and the protective insulating layer 203. The stackof the insulating layer 216 and the protective insulating layer 203 hasa plurality of opening portions overlapping with the common potentialline 205. This opening portion is formed by using the same step as thecontact hole that connects the drain electrode layer of the thin filmtransistor 220 to the pixel electrode layer 227.

Note that because of a significant difference in area, a distinction ismade here between the contact hole in the pixel portion and the openingportion in the common connection portion. Further, in FIG. 5A, the pixelportion and the common connection portion are not illustrated on thesame scale. For example, the length of the dashed line C5-C6 in thecommon connection portion is about 500 μm while the width of the thinfilm transistor is less than 50 μm thus, the common connection portionactually has an area ten times larger or more than an area of the thinfilm transistor. However, for simplicity, the pixel portion and thecommon connection portion are shown on different scales in FIG. 5A.

A common electrode layer 206 is provided over the stack of theinsulating layer 216 and the protective insulating layer 203, and formedby using the same material and step as the pixel electrode layer 227 inthe pixel portion.

In this manner, the common connection portion is manufactured in thesame forming step as the switching element in the pixel portion.

The first substrate provided with the pixel portion and the commonconnection portion and the second substrate having the counter electrodeare fixed with the sealant.

When the sealant is made to contain conductive particles, the pair ofsubstrates are aligned so that the sealant overlaps with the commonconnection portion. For example, in a small liquid crystal panel, twocommon connection portions are arranged so as to overlap with thesealant at opposite corners of the pixel portion and the like. In alarge liquid crystal panel, four or more common connection portions arearranged so as to overlap with the sealant.

Note that the common electrode layer 206 is an electrode in contact withthe conductive particles contained in the sealant, and is electricallyconnected to the counter electrode of the second substrate.

When a liquid crystal injection method is used, the two substrates arefixed with the sealant, and then liquid crystals are injected betweenthe pair of substrates. Alternatively, when a liquid crystal droppingmethod is used, the sealant is drawn on the second substrate or thefirst substrate, liquid crystals are dropped thereon, and then the twosubstrates are bonded to each other under a reduced pressure.

An example of the common connection portion electrically connected tothe counter electrode is described in this embodiment, but without anylimitation thereto, such a common connection portion can be used as aconnection portion connected to any other wiring or an externalconnection terminal or the like.

This embodiment can be combined with any one of Embodiments 1 to 3 asappropriate.

Embodiment 5

An example in which the gate insulating layer has a single-layerstructure is described in Embodiment 1 or Embodiment 2. In Embodiment 5,an example of a stacked structure will be described. Note thatcomponents common to FIGS. 6A and 6B and FIG. 3A or FIG. 3B keep thesame reference numerals.

In FIG. 6A, a thin film transistor 280 is an example of a channelprotective thin film transistor provided in a pixel portion, in which agate insulating layer has a two-layer structure. Note that except forthe two-layer structure of the gate electrode layer, the thin filmtransistor 280 is the same as the thin film transistor 220.

In this embodiment, the gate insulating layer has a stacked structure ofa first gate insulating layer 282 a having a thickness greater than orequal to 50 nm and less than or equal to 200 nm and a second gateinsulating layer 282 b having a thickness greater than or equal to 50 nmand less than or equal to 300 nm. As the first gate insulating layer 282a, a silicon nitride film or a silicon nitride oxide film having athickness of 100 nm is used. As the second gate insulating layer 282 b,a silicon oxide film having a thickness of 100 nm is used.

Note that a storage capacitor is provided below the pixel electrodelayer 227, and the capacitor electrode 231 is electrically connected tothe pixel electrode layer 227.

In this embodiment, the storage capacitor is formed using the capacitorelectrode 231 and the capacitor wiring layer 230.

In FIG. 6A, the storage capacitor includes only the gate insulatinglayer between the capacitor wiring and the capacitor electrode so as tohave large capacitance.

This embodiment shows an example in which a silicon oxide film formed bya sputtering method is used as an oxide insulating layer 282 b, and thesecond gate insulating layer formed using a silicon oxide film is etchedto be thin when the oxide insulating layer overlapping with thecapacitor wiring layer 230 is removed, whereby a third gate insulatinglayer 282 c is formed. Note that the first gate insulating layer 282 ais formed using a silicon nitride film or a silicon nitride oxide film,and functions as an etching stopper to prevent etching damage on thegate electrode layer or the substrate.

When the third gate insulating layer 282 c has a small thickness,storage capacitance can be increased.

FIG. 6B illustrates a cross-sectional structure which is partlydifferent from that in FIG. 6A.

In a thin film transistor 290 illustrated in FIG. 61, a gate insulatinglayer has a stacked structure of a first gate insulating layer 292 ahaving a thickness greater than or equal to 50 nm and less than or equalto 200 nm and a second gate insulating layer 292 b having a thicknessgreater than or equal to 1 nm and less than or equal to 50 nm. As thefirst gate insulating layer 292 a, a silicon oxide film having athickness of 100 nm is used. As the second gate insulating layer 292 b,a silicon nitride film or a silicon nitride oxide film having athickness of 10 nm is used.

The thin film transistor 290 is an example of a channel protective thinfilm transistor provided in a pixel portion, in which the gateinsulating layer has a two-layer structure. Note that except for thetwo-layer structure of the gate electrode layer, the thin filmtransistor 290 is the same as the thin film transistor 220.

This embodiment can be freely combined with any one of Embodiments 1 to4.

Embodiment 6

In this embodiment, an example of a thin film transistor whosemanufacturing process is partly different from that of Embodiment 1 willbe described with reference to FIGS. 7A to 7C and FIGS. 8A to 8E. FIGS.7A to 7C and FIGS. 8A to 8E are the same as FIGS. 1A to 1C and FIGS. 2Ato 2E except for a difference in part of the process. Therefore,identical components are designated by the same reference numerals, andnot described in detail again.

First, in accordance with Embodiment 1, a gate electrode layer, a gateinsulating layer, and the oxide semiconductor film 430 are formed over asubstrate; thus, steps up to and including the step shown FIG. 2A ofEmbodiment 1 are carried out. FIG. 2A is the same as FIG. 8A.

Then, the oxide semiconductor film 430 is processed into island-shapedoxide semiconductor layers by a second photolithography step.

Next, dehydration or dehydrogenation of the oxide semiconductor layer isperformed. The temperature of first heat treatment for dehydration ordehydrogenaion is set to greater than or equal to 400° C., and less thanthe strain point of the substrate, preferably 425° C. or more. Note thatin the case of the temperature of 425° C. or more, the heat treatmenttime may be one hour or less, whereas in the case of the temperatureless than 425° C., the heat treatment time is longer than one hour.Here, the substrate is introduced into an electric furnace, which is oneof heat treatment apparatuses, and heat treatment of the oxidesemiconductor layer is performed in a nitrogen atmosphere. Then, theoxide semiconductor layer is not exposed to air, which prevents furtherwater or hydrogen contamination, so that an oxide semiconductor layer isobtained. After that, cooling is performed by introduction of ahigh-purity oxygen gas, a high-purity N₂O gas, or ultra-dry air (havinga dew point of −40° C. or less, preferably −60° C. or less) into thesame furnace. It is preferable that the oxygen gas and the N₂O gas donot include water, hydrogen, and the like. Alternatively, the purity ofan oxygen gas or an N₂O gas which is introduced into the heat treatmentapparatus is preferably 6N (99.9999%) or more, more preferably 7N(99.99999%) or more (i.e., the impurity concentration of the oxygen gasor the N₂O gas is 1 ppm or less, preferably 0.1 ppm or less).

Further, after the first heat treatment for dehydration ordehydrogenation, heat treatment may be performed at greater than orequal to 200° C., and less than or equal to 400° C., preferably greaterthan or equal to 200° C., and less than or equal to 300° C., in anatmosphere of an oxygen gas or an N₂O gas.

Alternatively, the first heat treatment for the oxide semiconductorlayer can be performed with respect to the oxide semiconductor film 430before it is processed into the island-shaped oxide semiconductorlayers. In that case, after the first heat treatment, the substrate istaken out of the heating apparatus and a photolithography step isperformed.

Through the above process, the whole oxide semiconductor film is placedin an oxygen-excess state to have higher resistance, that is, becomes ani-type oxide semiconductor film.

Next, an oxide insulating film is formed over the gate insulating layer402 and the oxide semiconductor layer by a sputtering method. Then, aresist mask is formed by a third photolithography step, and the oxideinsulating layers 426 a and 426 b are formed by selective etching. Afterthat, the resist mask is removed (see FIG. 8B).

Next, a conductive film is formed over the gate insulating layer 402,the oxide insulating layers 426 a and 426 b, and an oxide semiconductorlayer 422. Then, a resist mask is formed by a fourth photolithographystep, and the source electrode layer 425 a and the drain electrode layer425 b are formed by selective etching (see FIG. 8C).

Next, in order to reduce variation in electric characteristics of thethin film transistors, heat treatment (preferably at greater than orequal to 150° C., and less than 350° C.) may be performed in an inertgas atmosphere or a nitrogen gas atmosphere. For example, heat treatmentis performed at 250° C. for 1 hour in a nitrogen atmosphere.

Next, a stack of an insulating layer 428 and a protective insulatinglayer 403 is formed over the oxide insulating layers 426 a and 426 b,the source electrode layer 425 a, and the drain electrode layer 425 b.

Next, a planarizing insulating layer 404 is formed over the protectiveinsulating layer 403.

Next, a fifth photolithography step is performed. A resist mask isformed, and the planarizing insulating layer 404, the protectiveinsulating layer 403, and the insulating layer 428 are etched, so thatthe contact hole 441 which reaches the drain electrode layer 425 b isformed. Then, the resist mask is removed (see FIG. 8D).

Next, a light-transmitting conductive film is formed.

Next, a sixth photolithography step is performed. A resist mask isformed, and unnecessary portion is removed by etching, so that the pixelelectrode layer 427 is formed. Then, the resist mask is removed (seeFIG. 8E).

Through the above process, a thin film transistor 420 and the wiringintersection portion with reduced parasitic capacitance can bemanufactured over one substrate with the use of six masks.

The thin film transistor 420 used for a pixel is a channel protectivethin film transistor which includes the oxide semiconductor layer 422having a channel formation region.

In addition, FIG. 7A is a plan view of the channel protective thin filmtransistor 420 which is provided in a pixel, and FIG. 7B is across-sectional view taken along line D7-D8 and line D11-D12 of FIG. 7A.FIG. 7C is a cross-sectional view taken along line D9-D10 of FIG. 7A.Note that FIG. 8E is the same as FIG. 7B.

This embodiment can be freely combined with any one of Embodiments 1 to5.

Embodiment 7

In this embodiment, an example of a structure of a storage capacitor,which is different from that of Embodiment 2, will be described withreference to FIGS. 9A and 9B. FIG. 9A is the same as FIG. 3A except fora difference in the structure of a storage capacitor. Therefore,identical components are designated by the same reference numerals andnot described in detail again. Note that FIG. 9A illustrates across-sectional structure of the thin film transistor 220 in a pixelportion and the storage capacitor.

FIG. 9A illustrates an example in which the storage capacitor is formedof the pixel electrode layer 227 and a capacitor wiring layer 250 whichoverlaps with the pixel electrode layer 227, with the use of theprotective insulating layer 203 and the planarizing insulating layer 204as a dielectric. The capacitor wiring layer 250 is formed using amaterial that is different from the material of the drain electrodelayer of the thin film transistor 220 in the pixel portion. The materialof the capacitor wiring layer 250 is also different from the material ofthe oxide semiconductor layer of the thin film transistor 220. Thecapacitor wiring layer 250 is formed using a light-transmittingconductive film. Note that, in addition to the photomasks used inEmbodiment 2, one more photomask for patterning of the capacitor wiringlayer 250 is used. Etching for forming the capacitor wiring layer 250 isperformed under conditions in which the exposed oxide semiconductorlayer of the thin film transistor 220 is not removed.

In the storage capacitor illustrated in FIG. 9A, the pair of electrodesand the dielectric have a light-transmitting property, and thus thewhole storage capacitor has a light-transmitting property. By making thestorage capacitor have a light-transmitting property, improvement inaperture ratio of the pixel portion can be achieved.

FIG. 9B illustrates an example of a storage capacitor having a structuredifferent from that in FIG. 9A. FIG. 9B is also the same as FIG. 3Aexcept for a difference in the structure of the storage capacitor.Therefore, identical components are designated by the same referencenumerals and not described in detail again.

FIG. 9B illustrates an example in which a storage capacitor is formed ofthe capacitor wiring layer 230 and a stack of an oxide semiconductorlayer 251 and the capacitor electrode 231 which overlap with thecapacitor wiring layer 230 with the use of the gate insulating layer 202as a dielectric. Further, the capacitor electrode 231 is stacked aboveand in contact with the oxide semiconductor layer 251 and functions asone of electrodes of the storage capacitor. Note that the oxidesemiconductor layer 251 is formed using the same material and in thesame forming step as the oxide semiconductor layer of the thin filmtransistor 220. The capacitor wiring layer 230 is formed using the samematerial and in the forming same step as the gate electrode layer of thethin film transistor 220; therefore, the capacitor wiring layer 230 isdisposed so that it does not overlap with a gate wiring layer of thethin film transistor 220. In addition, the capacitor electrode 231 iselectrically connected to the pixel electrode layer 227.

The material of the capacitor wiring layer 230 is also different fromthe material of the oxide semiconductor layer of the thin filmtransistor 220. The capacitor electrode 231 is formed using alight-transmitting conductive film. Note that in addition to thephotomasks used in Embodiment 2, one more photomask for patterning ofthe capacitor electrode 231 is used. Etching for forming the capacitorelectrode 231 is performed under conditions in which the exposed oxidesemiconductor layer of the thin film transistor 220 is not removed.

Also in the storage capacitor illustrated in FIG. 9B, the pair ofelectrodes and the dielectric have a light-transmitting property, andthus the whole storage capacitor has a light-transmitting property.

The storage capacitors illustrated in FIGS. 9A and 9B have alight-transmitting property, and high definition of a displayed image isachieved by increasing the number of gate wirings, for example.Therefore, even when the pixel size is reduced, sufficient capacitanceand a high aperture ratio can be obtained.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 8

In this embodiment, an example will be described below in which at leastsome of drive circuits and a thin film transistor placed in a pixelportion are formed over one substrate.

The thin film transistor placed in the pixel portion is formed asdescribed in any of Embodiments 1, 2, 5, and 6. Since the thin filmtransistor described in any of Embodiments 1, 2, 5, and 6 is ann-channel TFT, some of drive circuits that can be constituted byn-channel TFTs among the drive circuits are formed over the substratewhere the thin film transistor in the pixel portion is formed.

FIG. 14A illustrates an example of a block diagram of an active matrixdisplay device. A pixel portion 5301, a first scan line drive circuit5302, a second scan line drive circuit 5303, and a signal line drivecircuit 5304 are provided over a substrate 5300 in the display device.In the pixel portion 5301, a plurality of signal lines extended from thesignal line drive circuit 5304 are placed, and a plurality of scan linesextended from the first scan line drive circuit 5302 and the second scanline drive circuit 5303 are placed. Note that in crossing regions of thescan lines and the signal lines, pixels each having a display elementare arranged in a matrix. The substrate 5300 of the display device isconnected to a timing control circuit 5305 (also referred to as acontroller or a control IC) through a connection portion such as aflexible printed circuit (FPC).

In FIG. 14A, the first scan line drive circuit 5302, the second scanline drive circuit 5303, and the signal line drive circuit 5304 areformed over the substrate 5300 where the pixel portion 5301 is formed.Consequently, the number of components of a drive circuit and the likewhich are externally provided is reduced, so that costs can be reduced.Moreover, the number of connections in the connection portion which areformed when wirings are extended from a drive circuit provided outsidethe substrate 5300 can be reduced, and the reliability or yield can beincreased.

Note that the timing control circuit 5305 supplies, for example, a firstscan line drive circuit start signal (GSP1) and a scan line drivecircuit clock signal (GCK1) to the first scan line drive circuit 5302.Furthermore, the timing control circuit 5305 supplies, for example, asecond scan line drive circuit start signal (GSP2) (which is alsoreferred to as a start pulse) and a scan line drive circuit clock signal(GCK2) to the second scan line drive circuit 5303. Moreover, the timingcontrol circuit 5305 supplies a signal line drive circuit start signal(SSP), a signal line drive circuit clock signal (SCK), video signal data(DATA, also simply referred to as a video signal), and a latch signal(LAT) to the signal line drive circuit 5304. Each clock signal may be aplurality of clock signals with shifted phases or may be suppliedtogether with a signal (CKB) obtained by inverting the clock signal.Note that it is possible to omit one of the first scan line drivecircuit 5302 and the second scan line drive circuit 5303.

FIG. 14B illustrates a structure in which circuits with lower drivingfrequency (e.g., the first scan line drive circuit 5302 and the secondscan line drive circuit 5303) are formed over the substrate 5300 wherethe pixel portion 5301 is formed, and the signal line drive circuit 5304is formed over a substrate which is different from the substrate 5300where the pixel portion 5301 is formed. With this structure, the drivecircuits formed over the substrate 5300 can be constituted by thin filmtransistors whose field effect mobility is lower than that oftransistors including a single crystal semiconductor. Thus, an increasein size of the display device, a reduction in costs, improvement inyield, or the like can be achieved.

The thin film transistors in any of Embodiments 1, 2, 5, and 6 aren-channel TFTs. FIGS. 15A and 15B illustrate an example of a structureand operation of a signal line drive circuit constituted by n-channelTFTs.

The signal line drive circuit includes a shift register 5601 and aswitching circuit 5602. The switching circuit 5602 includes a pluralityof switching circuits 5602_1 to 5602_N (N is a natural number). Theswitching circuits 5602_1 to 5602_N each include a plurality of thinfilm transistors 56031 to 5603_k (k is a natural number). The examplewhere the thin film transistors 5603_1 to 5603_k are n-channel TFTs isdescribed below.

A connection relation in the signal line drive circuit is described byusing the switching circuit 5602_1 as an example. First terminals of thethin film transistors 5603_1 to 5603_k are connected to wirings 5604_1to 5604 k, respectively. Second terminals of the thin film transistors5603_1 to 5603 k are connected to signal lines Si to Sk, respectively.Gates of the thin film transistors 5603_1 to 5603_k are connected to awiring 5605_1.

The shift register 5601 has the function of sequentially selecting theswitching circuits 5602_1 to 5602_N by sequentially outputting H-levelsignals (also referred to as H signals or signals at a high power supplypotential level) to wirings 56051 to 5605_N.

The switching circuit 5602_1 has the function of controlling conductionstates between the wirings 5604_1 to 5604_k and the signal lines SI toSk (electrical continuity between the first terminals and the secondterminals), i.e., function of controlling whether potentials of thewirings 5604_1 to 5604_k are supplied to the signal lines SI to Sk. Inthis manner, the switching circuit 5602I functions as a selector.Moreover, the thin film transistors 56031 to 5603 k have the function ofcontrolling conduction states between the wirings 5604_1 to 5604_k andthe signal lines SI to Sk, respectively, i.e., function of supplyingpotentials of the wirings 5604_1 to 5604_k to the signal lines SI to Sk,respectively. In this manner, each of the thin film transistors 56031 to5603_k functions as a switch.

The video signal data (DATA) are input to each of the wirings 5604_1 to5604_k. The video signal data (DATA) are often an analog signal thatcorresponds to an image signal or image data.

Next, the operation of the signal line drive circuit in FIG. 15A isdescribed with reference to a timing chart in FIG. 15B. FIG. 15Billustrates examples of signals Sout_1 to Sout_N and signals Vdata_1 toVdata_k. The signals Sout_1 to Sout_N are examples of output signalsfrom the shift register 5601. The signals Vdata_1 to Vdata_k areexamples of signals input to the wirings 5604_1 to 5604_k. Note that oneoperation period of the signal line drive circuit corresponds to onegate selection period in a display device. For example, one gateselection period is divided into periods T1 to TN. Each of the periodsT1 to TN is a period for writing the video signal data (DATA) into apixel in a selected row.

Note that signal waveform distortion and the like in each structureillustrated in drawings and the like in this embodiment are exaggeratedfor simplicity in some cases. Therefore, this embodiment is notnecessarily limited to the scale illustrated in the drawings and thelike.

In the periods T1 to TN, the shift register 5601 sequentially outputsH-level signals to the wirings 5605_1 to 5605_N. For example, in theperiod T1, the shift register 5601 outputs an H-level signal to thewiring 5605_1. Then, the thin film transistors 5603_1 to 5603_k areturned on, so that the wirings 5604_1 to 5604_k and the signal lines SIto Sk are brought into conduction. At this time, Data(S1) to Data(Sk)are input to the wirings 5604I to 5604_k, respectively. The Data(S1) toData(Sk) are written into pixels in a first to kth columns in theselected row through the thin film transistors 5603_1 to 5603_k,respectively. In such a manner, in the periods T1 to TN, the videosignal data (DATA) are sequentially written into the pixels in theselected row by k columns.

The video signal data (DATA) are written into pixels by a plurality ofcolumns as described above, whereby the number of video signal data(DATA) or the number of wirings can be reduced. Consequently, the numberof connections with an external circuit can be reduced. Moreover, thetime for writing can be extended when video signals are written intopixels by a plurality of columns; thus, insufficient writing of videosignals can be prevented.

Note that any of the circuits constituted by the thin film transistorsin any of Embodiments 1, 2, 5, and 6 can be used for the shift register5601 and the switching circuit 5602. In that case, the shift register5601 can be constituted by only n-channel transistors or only p-channeltransistors.

One embodiment of a shift register which is used for part of the scanline drive circuit and/or the signal line drive circuit is describedwith reference to FIGS. 16A to 16D and FIGS. 17A and 17B.

The scan line drive circuit includes a shift register. Additionally, thescan line drive circuit may include a level shifter, a buffer, or thelike in some cases. In the scan line drive circuit, a clock signal (CLK)and a start pulse signal (SP) are input to the shift register, so that aselection signal is generated. The selection signal generated isbuffered and amplified by the buffer, and the resulting signal issupplied to a corresponding scan line. Gate electrodes of transistors inpixels of one line are connected to the scan line. Since the transistorsin the pixels of one line have to be turned on at all once, a bufferthat can supply a large current is used.

The shift register includes a first to Nth pulse output circuits 10_1 to10_N (N is a natural number greater than or equal to 3) (see FIG. 16A).In the shift register illustrated in FIG. 16A, a first clock signal CK1,a second clock signal CK2, a third clock signal CK3, and a fourth clocksignal CK4 are supplied from a first wiring 11, a second wiring 12, athird wiring 13, and a fourth wiring 14, respectively, to the first toNth pulse output circuits 10_1 to 10_N. A start pulse SP1 (a first startpulse) is input from a fifth wiring 15 to the first pulse output circuit10_1. To the nth pulse output circuit 10_n of the second or subsequentstage (n is a natural number greater than or equal to 2 and less than orequal to N), a signal from the pulse output circuit of the precedingstage (such a signal is referred to as a preceding-stage signalOUT(n−1)(SR)) (n is a natural number greater than or equal to 2 and lessthan or equal to N) is input. To the first pulse output circuit 10_1, asignal from the third pulse output circuit 10_3 of the stage followingthe next stage is input. Similarly, to the nth pulse output circuit 10_nof the second or subsequent stage, a signal from the (n+2)th pulseoutput circuit (n+2) of the stage following the next stage (such asignal is referred to as a subsequent-stage signal OUT(n+2)(SR)) isinput. Thus, the pulse output circuits of the respective stages outputfirst output signals (OUT(1)(SR) to OUT(N)(SR)) to be input to the pulseoutput circuit of the subsequent stage and/or to the pulse outputcircuit of the stage before the preceding stage, and second outputsignals (OUT(1) to OUT(N)) to be input to another circuit or the like.Note that since the subsequent-stage signal OUT(n+2) is not input to thelast two stages of the shift register as illustrated in FIG. 16A, asecond start pulse SP2 and a third start pulse SP3 may be input to thepulse output circuits of the last two stages, for example.

Note that a clock signal (CK) is a signal that alternates between an Hlevel and an L level (also referred to as an L signal or a signal at lowpower supply potential level) at regular intervals. Here, the firstclock signal (CK1) to the fourth clock signal (CK4) are delayed by ¼cycle sequentially. In this embodiment, driving or the like of the pulseoutput circuit is controlled with the first to fourth clock signals(CK1) to (CK4). Note that the clock signal is also referred to as GCK orSCK in some cases depending on a drive circuit to which the clock signalis input; the clock signal is referred to as CK in the followingdescription.

A first input terminal 21, a second input terminal 22, and a third inputterminal 23 are electrically connected to any of the first to fourthwirings 11 to 14. For example, in the first pulse output circuit 10_1 inFIG. 16A, the first input terminal 21 is electrically connected to thefirst wiring 11, the second input terminal 22 is electrically connectedto the second wiring 12, and the third input terminal 23 is electricallyconnected to the third wiring 13. In the second pulse output circuit10_2, the first input terminal 21 is electrically connected to thesecond wiring 12, the second input terminal 22 is electrically connectedto the third wiring 13, and the third input terminal 23 is electricallyconnected to the fourth wiring 14.

Each of the first to Nth pulse output circuits 10_1 to 10_N includes thefirst input terminal 21, the second input terminal 22, the third inputterminal 23, a fourth input terminal 24, a fifth input terminal 25, afirst output terminal 26, and a second output terminal 27 (see FIG.16B). In the first pulse output circuit 10_1, the first clock signal CK1is input to the first input terminal 21; the second clock signal CK2 isinput to the second input terminal 22; the third clock signal CK3 isinput to the third input terminal 23; a start pulse is input to thefourth input terminal 24; a subsequent-stage signal OUT(3) is input tothe fifth input terminal 25: the first output signal OUT(I)(SR) isoutput from the first output terminal 26; and the second output signalOUT(1) is output from the second output terminal 27.

In the first to Nth pulse output circuits 10_1 to 10_N, the thin filmtransistor (TFT) having four terminals described in the above embodimentcan be used in addition to a thin film transistor having threeterminals. FIG. 16C illustrates the symbol of a thin film transistor 28having four terminals which is described in the above embodiment. Thesymbol of the thin film transistor 28 illustrated in FIG. 16C representsthe thin film transistor having four terminals which is described in anyof Embodiments 1, 2, 5, and 6 and is used in the drawings and the like.Note that in this specification, when a thin film transistor has twogate electrodes with a semiconductor layer therebetween, the gateelectrode below the semiconductor layer is called a lower gate electrodeand the gate electrode above the semiconductor layer is called an uppergate electrode. The thin film transistor 28 is an element which cancontrol electric current between an IN terminal and an OUT terminal witha first control signal G1 which is input to a lower gate electrode and asecond control signal G2 which is input to an upper gate electrode.

When an oxide semiconductor is used as a semiconductor layer including achannel formation region in a thin film transistor, the thresholdvoltage sometimes shifts in the positive or negative direction dependingon a manufacturing process. For that reason, the thin film transistor inwhich an oxide semiconductor is used for a semiconductor layer includinga channel formation region preferably has a structure with which thethreshold voltage can be controlled. The threshold voltage of the thinfilm transistor 28 illustrated in FIG. 16C can be controlled to be at adesired level by providing gate electrodes above and below a channelformation region of the thin film transistor 28 with a gate insulatingfilm interposed between the upper gate electrode and the channelformation region and between the lower gate electrode and the channelformation region, and by controlling a potential of the upper gateelectrode and/or a potential of the lower gate electrode.

Next, an example of a specific circuit configuration of the pulse outputcircuit illustrated in FIG. 16B will be described with reference to FIG.16D.

The pulse output circuit illustrated in FIG. 16D includes a first tothirteenth transistors 31 to 43 (see FIG. 16D). A signal or a powersupply potential is supplied to the first to thirteenth transistors 31to 43 from a power supply line 51 to which a first high power supplypotential VDD is supplied, a power supply line 52 to which a second highpower supply potential VCC is supplied, and a power supply line 53 towhich a low power supply potential VSS is supplied, in addition to thefirst to fifth input terminals 21 to 25, the first output terminal 26,and the second output terminal 27, which are described above. Therelation of the power supply potentials of the power supply lines inFIG. 16D is as follows: the first power supply potential VDD is greaterthan or equal to the second power supply potential VCC, and the secondpower supply potential VCC is higher than the third power supplypotential VSS. Note that the first to fourth clock signals (CK1) to(CK4) each alternate between an H level and an L level at regularintervals; the clock signal at the H level is VDD and the clock signalat the L level is VSS. By making the potential VDD of the power supplyline 51 higher than the potential VCC of the power supply line 52, apotential applied to a gate electrode of a transistor can be lowered, ashift in threshold voltage of the transistor can be reduced, anddeterioration of the transistor can be suppressed without an adverseeffect on the operation of the transistor. Note that the thin filmtransistor 28 with four terminals is preferably used as the firsttransistor 31 and the sixth to ninth transistors 36 to 39 among thefirst to thirteenth transistors 31 to 43. The first transistor 31 andthe sixth to ninth transistors 36 to 39 need to switch a potential of anode to which one electrode serving as a source or a drain is connecteddepending on a control signal of the gate electrode, and can reduce amalfunction of the pulse output circuit by quick response (sharp risingof on-current) to the control signal input to the gate electrode. Byusing the thin film transistor 28 with four terminals, the thresholdvoltage can be controlled, and a malfunction of the pulse output circuitcan be further reduced.

In FIG. 16D, the first terminal of the first transistor 31 iselectrically connected to the power supply line 51, a second terminal ofthe first transistor 31 is electrically connected to the first terminalof the ninth transistor 39, and gate electrodes (a lower gate electrodeand an upper gate electrode) of the first transistor 31 are electricallyconnected to the fourth input terminal 24. A first terminal of thesecond transistor 32 is electrically connected to the power supply line53, a second terminal of the second transistor 32 is electricallyconnected to the first terminal of the ninth transistor 39, and a gateelectrode of the second transistor 32 is electrically connected to agate electrode of the fourth transistor 34. A first terminal of thethird transistor 33 is electrically connected to the first inputterminal 21, and a second terminal of the third transistor 33 iselectrically connected to the first output terminal 26. A first terminalof the fourth transistor 34 is electrically connected to the powersupply line 53, and a second terminal of the fourth transistor 34 iselectrically connected to the first output terminal 26. A first terminalof the fifth transistor 35 is electrically connected to the power supplyline 53, a second terminal of the fifth transistor 35 is electricallyconnected to the gate electrode of the second transistor 32 and the gateelectrode of the fourth transistor 34, and a gate electrode of the fifthtransistor 35 is electrically connected to the fourth input terminal 24.A first terminal of the sixth transistor 36 is electrically connected tothe power supply line 52, a second terminal of the sixth transistor 36is electrically connected to the gate electrode of the second transistor32 and the gate electrode of the fourth transistor 34, and gateelectrodes (a lower gate electrode and an upper gate electrode) of thesixth transistor 36 are electrically connected to the fifth inputterminal 25. A first terminal of the seventh transistor 37 iselectrically connected to the power supply line 52, a second terminal ofthe seventh transistor 37 is electrically connected to a second terminalof the eighth transistor 38, and gate electrodes (a lower gate electrodeand an upper gate electrode) of the seventh transistor 37 areelectrically connected to the third input terminal 23. A first terminalof the eighth transistor 38 is electrically connected to the gateelectrode of the second transistor 32 and the gate electrode of thefourth transistor 34, and gate electrodes (a lower gate electrode and anupper gate electrode) of the eighth transistor 38 are electricallyconnected to the second input terminal 22. The first terminal of theninth transistor 39 is electrically connected to the second terminal ofthe first transistor 31 and the second terminal of the second transistor32, a second terminal of the ninth transistor 39 is electricallyconnected to a gate electrode of the third transistor 33 and a gateelectrode of the tenth transistor 40, and gate electrodes (a lower gateelectrode and an upper gate electrode) of the ninth transistor 39 areelectrically connected to the power supply line 52. A first terminal ofthe tenth transistor 40 is electrically connected to the first inputterminal 21, a second terminal of the tenth transistor 40 iselectrically connected to the second output terminal 27, and the gateelectrode of the tenth transistor 40 is electrically connected to thesecond terminal of the ninth transistor 39. A first terminal of theeleventh transistor 41 is electrically connected to the power supplyline 53, a second terminal of the eleventh transistor 41 is electricallyconnected to the second output terminal 27, and a gate electrode of theeleventh transistor 41 is electrically connected to the gate electrodeof the second transistor 32 and the gate electrode of the fourthtransistor 34. A first terminal of the twelfth transistor 42 iselectrically connected to the power supply line 53, a second terminal ofthe twelfth transistor 42 is electrically connected to the second outputterminal 27, and a gate electrode of the twelfth transistor 42 iselectrically connected to the gate electrodes (the lower gate electrodeand the upper gate electrode) of the seventh transistor 37. A firstterminal of the thirteenth transistor 43 is electrically connected tothe power supply line 53, a second terminal of the thirteenth transistor43 is electrically connected to the first output terminal 26, and a gateelectrode of the thirteenth transistor 43 is electrically connected tothe gate electrodes (the lower gate electrode and the upper gateelectrode) of the seventh transistor 37.

In FIG. 16D, a connection portion where the gate electrode of the thirdtransistor 33, the gate electrode of the tenth transistor 40, and thesecond terminal of the ninth transistor 39 are connected is referred toas a node A. A connection portion where the gate electrode of the secondtransistor 32, the gate electrode of the fourth transistor 34, thesecond terminal of the fifth transistor 35, the second terminal of thesixth transistor 36, the first terminal of the eighth transistor 38, andthe gate electrode of the eleventh transistor 41 are connected isreferred to as a node B.

FIG. 17A illustrates signals that are input to or output from the firstto fifth input terminals 21 to 25 and the first and second outputterminals 26 and 27 in the case where the pulse output circuitillustrated in FIG. 16D is applied to the first pulse output circuit10_1.

Specifically, the first clock signal CK1 is input to the first inputterminal 21; the second clock signal CK2 is input to the second inputterminal 22; the third clock signal CK3 is input to the third inputterminal 23; the start pulse is input to the fourth input terminal 24;the subsequent-stage signal OUT(3) is input to the fifth input terminal25; the first output signal OUT(1)(SR) is output from the first outputterminal 26; and the second output signal OUT(1) is output from thesecond output terminal 27.

Note that a thin film transistor is an element having at least threeterminals of a gate, a drain, and a source. The thin film transistor hasa semiconductor including a channel region formed in a regionoverlapping with the gate. Current that flows between the drain and thesource through the channel region can be controlled by controlling apotential of the gate. Here, since the source and the drain of the thinfilm 6 transistor may interchange depending on the structure, theoperating condition, and the like of the thin film transistor, it isdifficult to define which is a source or a drain. Therefore, a regionfunctioning as the source or the drain is not called the source or thedrain in some cases. In that case, for example, such regions may bereferred to as a first terminal and a second terminal.

Note that in FIG. 16D and FIG. 17A, a capacitor for performing bootstrapoperation by bringing the node A into a floating state may beadditionally provided. Furthermore, a capacitor having one electrodeelectrically connected to the node B may be additionally provided inorder to hold a potential of the node B.

FIG. 17B is a timing chart of a shift register including a plurality ofpulse output circuits illustrated in FIG. 17A. Note that when the shiftregister is included in a scan line drive circuit, a period 61 in FIG.17B corresponds to a vertical retrace period and a period 62 correspondsto a gate selection period.

Note that by providing the ninth transistor 39 in which the second powersupply potential VCC is applied to the gate as illustrated in FIG. 17A,the following advantages before and after bootstrap operation areobtained.

Without the ninth transistor 39 in which the second power supplypotential VCC is applied to the gate electrode, if a potential of thenode A is raised by bootstrap operation, a potential of the source whichis the second terminal of the first transistor 31 rises to a valuehigher than the first power supply potential VDD. Then, the firstterminal of the first transistor 31. i.e., the terminal on the powersupply line 51 side, comes to serve as a source of the first transistor31. Consequently, in the first transistor 31, a high bias voltage isapplied and thus significant stress is applied between the gate and thesource and between the gate and the drain, which might causedeterioration of the transistor. On the other hand, with the ninthtransistor 39 in which the second power supply potential VCC is appliedto the gate electrode, an increase in potential of the second terminalof the first transistor 31 can be prevented while the potential of thenode A is raised by bootstrap operation. That is, providing the ninthtransistor 39 can lower the level of a negative bias voltage appliedbetween the gate and the source of the first transistor 31. Thus, thecircuit configuration in this embodiment can reduce a negative biasvoltage applied between the gate and the source of the first transistor31, so that deterioration of the rust transistor 31 due to stress can besuppressed.

Note that the ninth transistor 39 can be provided anywhere as long asthe first terminal and the second terminal of the ninth transistor 39are connected between the second terminal of the first transistor 31 andthe gate of the third transistor 33. Further, when the shift registerincluding a plurality of pulse output circuits in this embodiment isincluded in a signal line drive circuit having a larger number of stagesthan a scan line drive circuit, the ninth transistor 39 can be omitted,which is advantageous in that the number of transistors is reduced.

Note also that an oxide semiconductor is used for semiconductor layersof the first to thirteenth transistors 31 to 43: thus, the off-currentof the thin film transistors can be reduced, the on-current and fieldeffect mobility can be increased, and the degree of deterioration of thetransistors can be reduced. As a result, a malfunction in the circuitcan be reduced. Moreover, the degree of deterioration of the transistorusing an oxide semiconductor which is caused by application of a highpotential to a gate electrode is smaller than that of deterioration of atransistor using amorphous silicon. Consequently, even when the firstpower supply potential VDD is supplied to the power supply line whichsupplies the second power supply potential VCC, the shift register canoperate similarly and the number of power supply lines between circuitscan be reduced; thus, the size of the circuit can be reduced.

Note that the shift register will achieve similar effect even when theconnection relation is changed so that a clock signal which is suppliedto the gate electrodes (the lower gate electrode and the upper gateelectrode) of the seventh transistor 37 through the third input terminal23 is a clock signal which is supplied to the gate electrodes (the lowergate electrode and the upper electrode) of the seventh transistor 37through the second input terminal 22, and that a clock signal which issupplied to the gate electrodes (the lower gate electrode and the uppergate electrode) of the eighth transistor 38 through the second inputterminal 22 is a clock signal which is supplied to the gate electrodes(the lower electrode and the upper electrode) of the eighth transistor38 through the third input terminal 23. In the shift registerillustrated in FIG. 17A, a state of the seventh transistor 37 and theeighth transistor 38 is changed so that both the seventh transistor 37and the eighth transistor 38 are on, then the seventh transistor 37 isof and the eighth transistor 38 is on, and then the seventh transistor37 and the eighth transistor 38 are off; thus, a fall in potential ofthe node B, which is caused by a fall in potentials of the second inputterminal 22 and the third input terminal 23, is caused twice by a fallin potential of the gate electrode of the seventh transistor 37 and afall in potential of the gate electrode of the eighth transistor 38. Onthe other hand, in the case where a state of the seventh transistor 37and the eighth transistor 38 in the shift register illustrated in FIG.17A is changed as in the period 61 in FIG. 17B so that both the seventhtransistor 37 and the eighth transistor 38 are on, then the seventhtransistor 37 is on and the eighth transistor 38 is off, and then theseventh transistor 37 and the eighth transistor 38 are off; the fall inpotential of the node B, which is caused by the fall in potentials ofthe second input terminal 22 and the third input terminal 23, is causedonly once by the fall in potential of the gate electrode of the eighthtransistor 38. Consequently, it is preferable to use the clock signalsupplied to the gate electrodes (the lower gate electrode and the uppergate electrode) of the seventh transistor 37 from the third inputterminal 23 and the clock signal supplied to the gate electrodes (thelower gate electrode and the upper gate electrode) of the eighthtransistor 38 from the second input terminal 22 in order to reducefluctuation in potential of the node B and thus to reduce noise.

In such a manner, an H-level signal is regularly supplied to the node Bin a period during which the potentials of the first output terminal 26and the second output terminal 27 are held at an L level; thus, amalfunction of the pulse output circuit can be suppressed.

Embodiment 9

By fabricating thin film transistors and using the thin film transistorsfor a pixel portion and for a driver circuit, a semiconductor devicehaving a display function (also referred to as a display device) can bemanufactured. Moreover, some or all of the drive circuits which includethe thin film transistors, can be formed over the same substrate as thepixel portion, whereby a system-on-panel can be obtained.

The display device includes a display element. As a display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes in its category an element whose luminance is controlled bycurrent or voltage, and specifically includes in its category aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, the display device may include a display mediumwhose contrast is changed by an electric effect, such as electronic ink.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. Furthermore, an element substrate,which is one embodiment before the display element is completed in amanufacturing process of the display device, is provided with a meansfor supplying current to the display element in each of a plurality ofpixels. Specifically, the element substrate may be in a state in whichonly a pixel electrode of the display element is formed, a state inwhich a conductive film to be a pixel electrode is formed but is notetched yet to form the pixel electrode, or any other states.

Note that a display device in this specification refers to an imagedisplay device or a light source (including a lighting device). Further,the display device also includes any of the following modules in itscategory: a module to which a connector such as a flexible printedcircuit (FPC), a tape automated bonding (TAB) tape, or a tape carrierpackage (TCP) is attached; a module having a TAB tape or a TCP at theend of which a printed wiring board is provided; and a module having anintegrated circuit (IC) that is directly mounted on a display element bya chip on glass (COG) method.

The appearance and a cross section of a liquid crystal display panel,which is one embodiment of a semiconductor device, will be describedwith reference to FIGS. 10A1, 10A2, and 10B. FIGS. 10A1 and 10A2 areplan views of panels in which thin film transistors 4010 and 4011 and aliquid crystal element 4013 are sealed between a first substrate 4001and a second substrate 4006 with a sealant 4005. FIG. 10B is across-sectional view along line M-N in FIGS. 10A1 and 10A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line drive circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line drive circuit 4004. Consequently, thepixel portion 4002 and the scan line drive circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. A signal line drivecircuit 4003 which is formed using a single crystal semiconductor filmor a polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofthe drive circuit which is separately formed, and a COG (Chip On Glass)method, a wire bonding method, a TAB (Tape Automated Bonding) method, orthe like can be used. FIG. 10A1 illustrates an example in which thesignal line drive circuit 4003 is mounted by a COG method. FIG. 10A2illustrates an example in which the signal line drive circuit 4003 ismounted by a TAB method.

The pixel portion 402 and the scan line drive circuit 4004 provided overthe first substrate 4001 include a plurality of thin film transistors.FIG. 10B illustrates the thin film transistor 4010 included in the pixelportion 4002 and the thin film transistor 4011 included in the scan linedrive circuit 4004, as an example. Insulating layers 4041 a, 4041 b,4042 a, 4042 b, 4020, and 4021 are provided over the thin filmtransistors 4010 and 4011.

Any of the highly reliable thin film transistors including the oxidesemiconductor layers which are described in Embodiments 1, 2, 5, and 6can be used as the thin film transistors 4010 and 4011. The thin filmtransistors 260 and 270 described in Embodiments 1, 2, 5, and 6 can beused as the thin film transistor 4011 for the drive circuit, and thethin film transistors 420, 448, 220, 280, and 290 can be used as thethin film transistor 4010 for a pixel. In this embodiment, the thin filmtransistors 4010 and 4011 are n-channel thin film transistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4011 for the drivecircuit. The conductive layer 4040 is provided at the positionoverlapping with the channel formation region of the oxide semiconductorlayer, whereby the amount of change in threshold voltage of the thinfilm transistor 4011 before and after the BT (Bias Temperature stress)test can be reduced. A potential of the conductive layer 4040 may be thesame or different from that of a gate electrode layer of the thin filmtransistor 4011. The conductive layer 4040 can also function as a secondgate electrode layer. Alternatively, the potential of the conductivelayer 4040 may be GND or 0 V. or the conductive layer 4040 may be in afloating state.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with one another corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are respectively provided with an insulating layer 4032 and aninsulating layer 4033 functioning as alignment films, and the liquidcrystal layer 4008 is sandwiched between the pixel electrode layer 4030and the counter electrode layer 4031 with the insulating layers 4032 and4033 therebetween.

Note that a light-transmitting substrate can be used as the firstsubstrate 4001 and the second substrate 4006; glass, ceramics, orplastics can be used. Examples of plastics that can be used include afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating film and provided in order to control the distance (a cellgap) between the pixel electrode layer 4030 and the counter electrodelayer 4031. Alternatively, a spherical spacer may be used. The counterelectrode layer 4031 is electrically connected to a common potentialline formed over the same substrate as the thin film transistor 4010.The counter electrode layer 4031 and the common potential line can beelectrically connected to each other through conductive particlesarranged between a pair of substrates using the common connectionportion. Note that the conductive particles are included in the sealant4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while the temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperature, a liquid crystal compositionincluding a chiral agent at 5 wt % or more is used for the liquidcrystal layer 4008 in order to improve the temperature range. The liquidcrystal composition that includes liquid crystal exhibiting a blue phaseand a chiral agent has a short response time of 1 msec or less and isoptically isotropic; therefore, alignment treatment is not necessary andviewing angle dependence is small.

Note that this embodiment can also be applied to a transflective liquidcrystal display device in addition to a transmissive liquid crystaldisplay device.

An example of the liquid crystal display device is described in which apolarizing plate is provided on the outer surface of the substrate (onthe viewer side) and a coloring layer and an electrode layer used for adisplay element are provided on the inner surface of the substrate inthis order; however, the polarizing plate may be provided on the innersurface of the substrate. The stacked-layer structure of the polarizingplate and the coloring layer is not limited to that in this embodimentand may be set as appropriate depending on materials of the polarizingplate and the coloring layer or conditions of the manufacturing process.Further, a light-blocking film serving as a black matrix may be providedin a portion other than the display portion.

In the thin film transistor 4011, the insulating layer 4041 a serving asa channel protective layer and the insulating layer 4041 b covering aperipheral portion (including a side surface) of the oxide semiconductorlayer are formed. In a similar manner, in the thin film transistor 4010,the insulating layer 4042 a serving as a channel protective layer andthe insulating layer 4042 b covering a peripheral portion (including aside surface) of the oxide semiconductor layer are formed.

The insulating layers 4041 b and 4042 b which cover the peripheralportion (including the side surfaces) of the oxide semiconductor layerincreases the distance between the gate electrode layer and a wiringlayer (e.g., a source wiring layer or a capacitor wiring layer) over orin the periphery of the gate electrode layer, whereby parasiticcapacitance can be reduced. The insulating layers 4041 a, 441 b, 4042 a,and 4042 b may be formed using the same material and method as the oxideinsulating layers 426 a and 426 b described in Embodiment 1. Inaddition, in order to reduce the surface roughness generated by the thinfilm transistors, the thin film transistors are covered with theinsulating layer 4021 serving as a planarizing insulating film. Here, asthe insulating layers 4041 a, 4041 b, 4042 a, and 4042 b, a siliconoxide film is formed by a sputtering method according to Embodiment 1.

The insulating layer 4020 is formed over the insulating layers 4041 a,4101 b, 4042 a, and 4042 b. The insulating layer 4020 can be formedusing the same material and method as the stack of the insulating layer428 and the protective insulating layer 403 described in Embodiment 1.The insulating layer 4020 is illustrated as a single layer in FIG. 10B,but is the stack of the insulating layer 428 and the protectiveinsulating layer 403 which are formed using different materials. Here,as the insulating layer 4020, a silicon oxide film formed by asputtering method and a silicon nitride film formed by a plasma CVDmethod are stacked.

The insulating layer 4021 is formed as the planarizing insulating film.The insulating layer 4021 can be formed using the same material andmethod as the planarizing insulating layer 404 described in Embodiment1, and a heat-resistant organic material such as polyimide, acrylicresin, benzocyclobutene-based resin, polyamide, or epoxy resin can beused. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating films formed from these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent. Theorganic group may include a fluoro group.

There is no particular limitation on the method of forming theinsulating layer 4021. Depending on the material, the insulating layer4021 can be formed by a method such as sputtering method, an SOG method,a spin coating method, a dipping method, a spray coating method, or adroplet discharge method (e.g., an inkjet method, screen printing, oroffset printing), or by using a tool (apparatus) such as a doctor knife,a roll coater, a curtain coater, a knife coater, or the like. When thebaking step of the insulating layer 4021 and the annealing of thesemiconductor layer are combined, a semiconductor device can bemanufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed from a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter, referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Alternatively, a conductive composition including a conductive highmolecular weight molecule (also referred to as a conductive polymer) canbe used for the pixel electrode layer 4030 and the counter electrodelayer 4031. The pixel electrode formed using a conductive compositionpreferably has a sheet resistance of 10000 Ω/square or less and a lighttransmittance of 70% or more at a wavelength of 550 nm. Further, theresistivity of the conductive high molecule included in the conductivecomposition is preferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called n-electron conjugatedconductive polymer can be used Examples are polyaniline and a derivativethereof, polypyrrole and a derivative thereof, polythiophene and aderivative thereof, and a copolymer of two or more of these materials.

Further, a variety of signals and potentials are supplied to the signalline drive circuit 4003 which is separately formed, the scan line drivecircuit 4004, or the pixel portion 4002 from an FPC 4018.

A connection terminal electrode 4015 is formed using the same conductivefilm as the pixel electrode layer 4030 included in the liquid crystalelement 4013. A terminal electrode 4016 is formed using the sameconductive film as source and drain electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 10A 1, 10A2, and 10B illustrate the example in which thesignal line drive circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, this embodiment is not limited to thisstructure. The scan line drive circuit may be separately formed and thenmounted, or only part of the signal line drive circuit or part of thescan line drive circuit may be separately formed and then mounted.

FIG. 19 illustrates an example of a liquid crystal display module whichis formed as a semiconductor device using a TFT substrate 2600manufactured according to the manufacturing method disclosed in thisspecification.

FIG. 19 illustrates an example of the liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTand the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the RGB system, coloring layers corresponding to colors ofred, green, and blue are provided for pixels. Polarizing plates 2606 and2607 and a diffusion plate 2613 are provided outside the TFT substrate2600 and the counter substrate 2601. Alight source includes a coldcathode tube 2610 and a reflective plate 2611. A circuit board 2612 isconnected to a wiring circuit portion 2608 of the TFT substrate 2600 bya flexible wiring board 2609 and includes an external circuit such as acontrol circuit or a power source circuit. The polarizing plate and theliquid crystal layer may be stacked with a retardation plate interposedtherebetween.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be employed.

Through the above, a highly reliable liquid crystal display panel as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 10

An example of an electronic paper will be described as an example of asemiconductor device.

The semiconductor device can be used for an electronic paper in whichelectronic ink is driven by an element electrically connected to aswitching element. The electronic paper is also called anelectrophoretic display device (an electrophoretic display) and hasadvantages in that it has the same level of readability as plain paper,it has less power consumption than other display devices, and it can bemade thin and lightweight.

There are a variety of modes of electrophoretic displays. Theelectrophoretic display includes a plurality of microcapsules dispersedin a solvent or a solute; each microcapsule containing first particleswhich are positively charged and second particles which are negativelycharged. When electric field is applied to the microcapsules, theparticles in the microcapsules move in opposite directions to each otherand only the color of the particles gathered on one side is displayed.Note that the first particles or the second particles include a pigment,and do not move without an electric field. The first particles and thesecond particles have different colors (which may be colorless).

Thus, the electrophoretic display utilires a so-called dielectrophoreticeffect, in which a substance with high dielectric constant moves to aregion with a high electric field.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. Electronic ink can be printed on asurface of glass, plastic, fabric, paper, or the like. Furthermore, byuse of a color filter or particles that have a pigment, color displaycan also be achieved.

In addition, if a plurality of the above microcapsules are arranged asappropriate over an active matrix substrate so as to be interposedbetween two electrodes, an active matrix display device can becompleted. Images can be displayed by application of an electric fieldto the microcapsules. For example, the active matrix substrate obtainedusing the thin film transistors described in Embodiments 1, 2, 5, and 6can be used.

Note that the first particles and the second particles in themicrocapsules may be formed from any one of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material, orformed from a composite material thereof.

FIG. 18 illustrates an active matrix electronic paper as an example of asemiconductor device. A thin film transistor 581 used for thesemiconductor device can be manufactured in a manner similar to that ofthe thin film transistor described in Embodiment 1 and is a highlyreliable thin film transistor including an oxide semiconductor layer.The thin film transistor described in any of Embodiments 2, 5 and 6 canalso be used as the thin film transistor 581 in this embodiment.

The electronic paper of FIG. 18 is an example of a display device inwhich a twisting ball display system is employed. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are arranged between a first electrode layerand a second electrode layer which are electrode layers used for adisplay element, and a potential difference is generated between thefirst electrode layer and the second electrode layer to controlorientation of the spherical particles, so that display is performed.

The thin film transistor 581 formed over a substrate 580 is abottom-gate thin film transistor and is covered with an insulating film583 which is in contact with a semiconductor layer. A source electrodelayer or a drain electrode layer of the thin film transistor 581 is incontact with a first electrode layer 587 through an opening formed in aninsulating layer 585, whereby the thin film transistor 581 iselectrically connected to the first electrode layer 587. Between thefirst electrode layer 587 and a second electrode layer 588 formed on asubstrate 596, spherical particles 589 each having a black region 590 a,a white region 590 b, and a cavity 594 around the regions which isfilled with liquid are provided. Space around the spherical particles589 is filled with a filler 595 such as a resin. The first electrodelayer 587 corresponds to a pixel electrode, and the second electrodelayer 588 corresponds to a common electrode. The second electrode layer588 is electrically connected to a common potential line provided overthe same substrate as the thin film transistor 581. With the use of acommon connection portion, the second electrode layer 588 can beelectrically connected to the common potential line through conductiveparticles provided between the pair of substrates.

Instead of the twisting ball, an electrophoretic element can be used. Amicrocapsule having a diameter of about 10 mm to 200 mm in whichtransparent liquid, positively-charged white microparticles, andnegatively-charged black microparticles are encapsulated, is used. Inthe microcapsules which are provided between the first electrode layerand the second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and isgenerally called an electronic paper. The electrophoretic displayelement has higher reflectance than a liquid crystal display element,and thus, an auxiliary light is unnecessary, power consumption is low,and a display portion can be recognized in a dim place. In addition,even when power is not supplied to the display portion, an image whichhas been displayed once can be maintained. Accordingly, a displayedimage can be stored even if a semiconductor device having a displayfunction (which may be referred to simply as a display device or asemiconductor device provided with a display device) is distanced froman electric wave source.

Through the above, a highly reliable electronic paper as a semiconductordevice can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 11

An example of a light-emitting display device will be described as asemiconductor device. As a display element of the display device, alight-emitting element utilizing electroluminescence is described here.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, those carriers (i.e., electrons and holes) arerecombined, and thus, the light-emitting organic compound is excited.When the light-emitting organic compound relaxes to the ground statefrom the excited state, light is emitted. Owing to such a mechanism,this light-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIG. 12 illustrates an example of a pixel structure as an example of asemiconductor device which can be driven by a digital time grayscalemethod.

The structure and operation of a pixel which can be driven by a digitaltime grayscale method will be described. In this example, one pixelincludes two n-channel transistors using an oxide semiconductor layer ina channel formation region.

A pixel 6400 includes a switching transistor 6401, a light-emittingelement driving transistor 6402, a light-emitting element 64M, and acapacitor 6403. A gate of the switching transistor 6401 is connected toa scan line 6406, a first electrode (one of a source electrode and adrain electrode) of the switching transistor 6401 is connected to asignal line 6405, and a second electrode (the other of the sourceelectrode and the drain electrode) of the switching transistor 6401 isconnected to a gate of the light-emitting element driving transistor6402. The gate of the light-emitting element driving transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driving transistor 6402 is connected to the powersupply line 6407, and a second electrode of the light-emitting elementdriving transistor 6402 is connected to a first electrode (pixelelectrode) of the light-emitting element 6404. A second electrode of thelight-emitting element 6404 corresponds to a common electrode 6408. Thecommon electrode 6408 is electrically connected to a common potentialline provided over the same substrate.

Note that the second electrode (the common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Notethat the low power supply potential is a potential less than a highpower supply potential which is supplied to the power supply line 6407.For example, GND or 0 V may be set as the low power supply potential.The difference between the high power supply potential and the low powersupply potential is applied to the light-emitting element 6404 so thatcurrent flows through the light-emitting element 6404, whereby thelight-emitting element 6404 emits light. Thus, each potential is set sothat the difference between the high power supply potential and the lowpower supply potential is greater than or equal to a forward thresholdvoltage of the light-emitting element 6404.

When the gate capacitance of the light-emitting element drivingtransistor 6402 is used as a substitute for the capacitor 6403, thecapacitor 6403 can be omitted. The gate capacitance of thelight-emitting element driving transistor 6402 may be formed between achannel region and a gate electrode.

Here, in the case of employing a voltage-input voltage driving method, avideo signal is input to the gate of the light-emitting element drivingtransistor 6402 to make the light-emitting element driving transistor6402 completely turn on or off. I.e., the light-emitting element drivingtransistor 6402 operates in a linear region, and thus, a voltage higherthan the voltage of the power supply line 6407 is applied to the gate ofthe light-emitting element driving transistor 6402. Note that a voltagegreater than or equal to (power supply line voltage +V_(th) of thelight-emitting element driving transistor 6402) is applied to the signalline 6405.

In the case of employing an analog grayscale method instead of thedigital time grayscale method, the same pixel structure as in FIG. 12can be employed by inputting signals in a different way.

In the case of employing the analog grayscale method, a voltage greaterthan or equal to (forward voltage of the light-emitting element6404+V_(th) of the light-emitting element driving transistor 6402) isapplied to the gate of the light-emitting element driving transistor6402. The forward voltage of the light-emitting element 6404 refers to avoltage to obtain a desired luminance, and includes at least a forwardthreshold voltage. By inputting a video signal to enable thelight-emitting element driving transistor 6402 to operate in asaturation region, current can be supplied to the light-emitting element6404. In order that the light-emitting element driving transistor 6402can operate in the saturation region, the potential of the power supplyline 6407 is set higher than a gate potential of the light-emittingelement driving transistor 6402. Since the video signal is an analogsignal, current in accordance with the video signal flows in thelight-emitting element 6404, and analog grayscale can be performed.

Note that the pixel structure is not limited to that illustrated in FIG.12. For example, the pixel in FIG. 12 can further include a switch, aresistor, a capacitor, a transistor, a logic circuit, or the like.

Next, structures of the light-emitting element will be described withreference to FIGS. 13A to 13C. Here, a cross-sectional structure of apixel will be described by taking an n-channel TFT as an example. TFTs7001, 7011, and 7021 which are TFTs used for semiconductor devicesillustrated in FIGS. 13A to 13C can be manufactured in a manner similarto that of the thin film transistor described in Embodiment 1. The TFTs7001, 7011, and 7021 are highly reliable thin film transistors eachincluding an oxide semiconductor layer. Alternatively, the thin filmtransistor described in any of Embodiments 2, 5, and 6 can be employedas the driving TFTs 7001, 7011, and 7021.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode should be transparent. There arefollowing structures of a light-emitting element which is formed overthe same substrate as a thin film transistor: a top-emission structurein which light is extracted through the surface opposite to thesubstrate, a bottom-emission structure in which light is extractedthrough the surface of the substrate, and a dual-emission structure inwhich light is extracted through the surface opposite to the substrateand the surface of the substrate. The pixel structure can be applied toa light-emitting element having any of these emission structures.

A light-emitting element having a top-emission structure will bedescribed with reference to FIG. 13A.

FIG. 13A is a cross-sectional view of a pixel in the case where the TFT7001 is of an n-type and light is emitted from a light-emitting element7002 through an anode 7005. In FIG. 13A, a cathode 7003 of thelight-emitting element 7002 is electrically connected to the TFT 7001via a contact hole made through a planarizing insulating layer 7007, aprotective-insulating layer 7000 and a insulating layering 7006, apartition wall 7009 is formed over the contact hole, and alight-emitting layer 7004 and the anode 7005 are stacked in this orderover the cathode 7003. The cathode 7003 can be formed using any ofconductive materials which have a low work function and which reflectslight. For example. Ca, Al. CaF, Mg, Ag, AILi, or the like is preferablyused. The light-emitting layer 704 may be formed as a single layer or astack of plural layers. When the light-emitting layer 70 t 4 is formedas a stack of plural layers, the light-emitting layer 7004 is formed bystacking an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerin this order over the cathode 7003. Note that not all of these layersneed to be provided. The anode 7005 is formed of a light-transmittingconductive material. For example, the anode 7005 may be formed using alight-transmitting conductive film of indium oxide including tungstenoxide, indium zinc oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide including titanium oxide,indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thelight-emitting layer 7004 is sandwiched between the cathode 7003 and theanode 7005. In the pixel illustrated in FIG. 13A, light is emitted fromthe light-emitting element 7002 to the anode 7005 side as indicated byan arrow.

Next, a light-emitting element having a bottom-emission structure willbe described with reference to FIG. 13B. FIG. 13B is a cross-sectionalview of a pixel in the case where the TFT 7011 for driving thelight-emitting element is of an n-type and light is emitted from alight-emitting element 7012 through a cathode 7013. In FIG. 13B, thecathode 7013 of the light-emitting element 7012 is formed over alight-transmitting conductive film 7017 which is electrically connectedto the TFT 7011 for driving the light-emitting element, and alight-emitting layer 7014 and an anode 7015 are stacked in this orderover the cathode 7013. Note that a blocking film 7016 for reflecting orblocking light may be formed so as to cover the anode 7015 when theanode 7015 has a light-transmitting property. For the cathode 7013, anyof conductive materials which have a low work function can be used as inthe case of FIG. 13A. Note that the cathode 7013 is formed to athickness with which the cathode 7013 transmits light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film having athickness of 20 nm can be used as the cathode 7013. Similarly to thecase of FIG. 13A, the light-emitting layer 7014 may be formed usingeither a single layer or a stack of plural layers. The anode 7015 is notrequired to transmit light, but can be formed of a light-transmittingconductive material as in the case of FIG. 13A. As the blocking film7016, a metal film which reflects light can be used for example;however, it is not limited to a metal film. For example, a resin towhich a black pigment is added can also be used.

The light-emitting element 7014 corresponds to a region where thelight-emitting layer 7014 is sandwiched between the cathode 7013 and theanode 7015. In the pixel illustrated in FIG. 13B, light is emitted fromthe light-emitting element 7012 to the cathode 7013 side as indicated byan arrow. Note that FIG. 13B illustrates an example in which alight-transmitting conductive film is used as the gate electrode layer,so that light is emitted from the light-emitting element 7012 throughthe gate electrode layer.

Next, a light-emitting element having a dual-emission structure will bedescribed with reference to FIG. 13C. In FIG. 13C, the cathode 7023 ofthe light-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the TFT 7021 fordriving the light-emitting element, and a light-emitting layer 7024 andan anode 7025 are stacked in this order over the cathode 7023. For thecathode 7023, any of conductive materials which have a low work functioncan be used as in the case of FIG. 13A. Note that the cathode 7023 isformed to a thickness with which the cathode 7023 transmits light. Forexample, a 20-nm-thick Al film can be used as the cathode 7023.Similarly to the case of FIG. 13A, the light-emitting layer 7024 may beformed using either a single layer or a stack of plural layers. Theanode 7025 can be formed of a light-transmitting conductive material asin the case of FIG. 13A.

The light-emitting element 7022 corresponds to a region where the anode7025, the light-emitting layer 7024 and the cathode 7023 overlap witheach other. In the pixel illustrated in FIG. 13C, light is emitted fromthe light-emitting element 7022 to both the anode 7025 side and thecathode 7023 side as indicated by an arrow.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can alternatively be provided as alight-emitting element.

Although the example in which a thin film transistor (a TFT for drivinga light-emitting element) which controls the driving of a light-emittingelement is electrically connected to the light-emitting element has beendescribed, a structure may be employed in which a TFT for currentcontrol is connected between the TFT and the light-emitting element.

Note that the structure of the semiconductor device is not limited tothose illustrated in FIGS. 13A to 13C and can be modified in variousways based on the spirit of techniques according to the presentinvention.

Next, an appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which is oneembodiment of the semiconductor device, will be described with referenceto FIGS. 11A and 11B. FIG. 11A is a plan view of a panel in which a thinfilm transistor and a light-emitting element are sealed between a firstsubstrate and a second substrate with a sealant. FIG. 11B is across-sectional view taken along line H-I of FIG. 11A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line drive circuits 4503 a and 4503 b, and scan line drivecircuits 4504 a and 4504 b, which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line drive circuits 4503 a and 4503 b, and thescan line drive circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line drive circuits 4503 a and 4503 b, and thescan line drive circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be thus packaged(sealed) with a protective film (such as a bonding film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air.

The pixel portion 4502, the signal line drive circuits 4503 a and 4503b, and the scan line drive circuits 4504 a and 4504 b which are formedover the first substrate 4501 each include a plurality of thin filmtransistors. A thin film transistor 4510 included in the pixel portion4502 and a thin film transistor 4509 included in the signal line drivecircuit 4503 a are illustrated as an example in FIG. 11B.

As the thin film transistors 4509 and 4510, the highly reliable thinfilm transistor including an oxide semiconductor layer which isdescribed in any of Embodiments 1, 2, 5, and 6 can be employed. The thinfilm transistors 4509 and 4510 are n-channel thin film transistors. Thethin film transistors 260 and 270 described in Embodiments 1, 2, 5, and6 can be used as the thin film transistor 450′) for the drive circuit,and the thin film transistors 420, 448, 220, 280, and 290 can be used asthe thin film transistor 4510 for a pixel. In this embodiment, the thinfilm transistors 4509 and 4510 are n-channel thin film transistors.

A conductive layer 4540 is provided over part of the insulating layer4544, which overlaps with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4509 for the drivecircuit. The conductive layer 4540 is provided at the positionoverlapping with the channel formation region of the oxide semiconductorlayer, whereby the variation in threshold voltage of the thin filmtransistor 4509 before and after the BT test can be reduced. A potentialof the conductive layer 4540 may be the same or different from that of agate electrode layer of the thin film transistor 4509. The conductivelayer 4540 can also function as a second gate electrode layer.Alternatively, the potential of the conductive layer 4540 may be GND or0 V. or the conductive layer 4540 may be in a floating state.

In the thin film transistor 4509, the insulating layer 4541 a serving asa channel protective layer and the insulating layer 4541 b which coversa peripheral portion (and the side surface) of the oxide semiconductorlayer are formed. In a similar manner, in the thin film transistor 4510,the insulating layer 4542 a serving as a channel protective layer andthe insulating layer 4542 b which covers a peripheral portion (and theside surface) of the oxide semiconductor layer are formed.

The insulating layers 4541 b and 4542 b which cover the peripheralportion (and the side surface) of the oxide semiconductor layerincreases the distance between the gate electrode layer and the wiringlayer (e.g., a source wiring layer or a capacitor wiring layer) over orin the periphery of the gate electrode layer, whereby parasiticcapacitance can be reduced. The insulating layers 4541 a, 4541 b, 4542a, and 4542 b may be formed of a material and a method similar to thematerial and method for the oxide insulating layers 426 a and 426 bdescribed in Embodiment 1. In addition, in order to reduce the surfaceroughness due to the thin film transistors, the thin film transistorsare covered with the insulating layer 4543 serving as a planarizinginsulating film. Here, as the insulating layers 4541 a. 4541 b, 4542 a,and 4542 b, a silicon oxide film is formed by a sputtering methodaccording to Embodiment 1.

The insulating layer 4543 is formed over the insulating layers 4541 a,4541 b, 4542 a, and 4542 b. The insulating layer 4543 can be formedusing the same material and method as the insulating layer 428 and theprotective insulating layer 403 described in Embodiment 1. Theinsulating layer 4543 is illustrated as a single layer in FIG. 10B, butis the stack of the insulating layer 428 and the protective insulatinglayer 403 which are formed using different materials. Here, as theinsulating layer 4543, a silicon oxide film formed by a sputteringmethod and a silicon nitride film formed by a plasma CVD method arestacked.

The insulating layer 4544 is formed as the planarizing insulating film.The insulating layer 4544 can be formed using the same material andmethod as the planarizing insulating layer 404 described inEmbodiment 1. Here, acrylic is used for the insulating layer 4544.

In this embodiment, a plurality of thin film transistors in the pixelportion may be surrounded together by a nitride insulating film. Forexample, a structure may be employed in which a nitride insulating filmis used as the insulating layer 4543 and the gate insulating layer, andthe insulating layer 4543 is in contact with the gate insulating layerat least in the periphery surrounding the pixel portion over the activematrix substrate as illustrated in FIGS. 11A and 11B. By employing abovestructure, entry of moisture from the outside can be prevented.Moreover, entry of moisture from the outside can be prevented in thelong term even after the device is completed as a semiconductor device,for example, as a display device: thus, the long-term reliability of thedevice can be improved.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that although the light-emitting element 4511 has astacked-layer structure including the first electrode layer 4517, anelectroluminescent layer 4512, and a second electrode layer 4513 in thisembodiment, the structure of the light-emitting element 4511 is notlimited. The structure of the light-emitting element 4511 can be changedas appropriate depending on, for example, the direction in which lightis extracted from the light-emitting element 4511.

A partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed of a photosensitivematerial to have an opening over the first electrode layer 4517 so thata sidewall of the opening is formed as an inclined surface with acontinuous curvature.

The electrical field light-emitting layer 4512 may be formed usingeither a single layer or a stack of plural layers.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide,or the like into the light-emitting element 4511, a protective film maybe formed over the second electrode layer 4513 and the partition wall4520. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC (Diamond-Like Carbon) film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs4518 a and 4518 b to the signal line drive circuits 4503 a and 4503 b,the scan line drive circuits 4504 a and 4504 b, or the pixel portion4502.

A connection terminal electrode 4515 is formed using the same conductivefilm as the first electrode layer 4517 included in the light-emittingelement 4511. A terminal electrode 4516 is formed using the sameconductive film as the source and drain electrode layers included in thethin Film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. For example, nitrogen may be used as the filler.

If needed, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate on an emission surface ofthe light-emitting element. Further, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions of the surface so as to reducethe glare can be performed.

As the signal line drive circuits 4503 a and 4503 b and the scan linedrive circuits 4504 a and 4504 b, drive circuits formed by using asingle crystal semiconductor film or a polycrystalline semiconductorfilm over a substrate separately prepared may be mounted. Alternatively,only the signal line drive circuits or part thereof, or only the scanline drive circuits or part thereof may be separately formed and thenmounted. This embodiment is not limited to the structure illustrated inFIGS. 11A and 11B.

Through the above steps, a highly reliable light-emitting device (adisplay panel) can be manufactured as a semiconductor device.

This embodiment can be implemented in combination with any of thestructures described in Embodiments 1 to 4 and 6 to 8, as appropriate.

Embodiment 12

The semiconductor device disclosed in this specification can be appliedto an electronic paper. An electronic paper can be used for electronicdevices for displaying information in all fields. For example, anelectronic paper can be applied to an electronic book (an e-book), aposter, an advertisement in a vehicle such as a train, or a display of avariety of cards such as a credit card. FIG. 20 illustrates an exampleof the electronic devices.

FIG. 20 illustrates an example of an electronic book 27M. For example,the electronic book 2700 includes two housings, a housing 2701 and ahousing 2703. The housing 2701 and the housing 2703 are combined with ahinge 2711 so that the electronic book 2700 can be opened and closedalong the hinge 2711. With such a structure, the electronic book 2700can be handled like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, a display portion onthe right (the display portion 2705 in FIG. 20) can display text and adisplay portion on the left (the display portion 2707 in FIG. 20) candisplay graphics.

FIG. 20 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power button 2721, operation keys 2723, a speaker 2725,and the like. Pages can be turned with the operation keys 2723. Notethat a keyboard, a pointing device, and the like may be provided on thesame surface as the display portion of the housing. Moreover, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal connectable to a variety of cables such as an AC adapter or aUSB cable), a storage medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing.Moreover, the electronic book 2700 may have the function of anelectronic dictionary.

The electronic book 270 may be configured to wirelessly transmit andreceive data. Through wireless communication, desired book data or thelike can be purchased and downloaded from an electronic book server.

Embodiment 13

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofsuch electronic devices are a television device (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game console, aportable information terminal, an audio playback device, a large-sizedgame machine such as a pinball machine, and the like.

FIG. 21A illustrates an example of a television device 9600. In thetelevision device 9600, a display portion 9603 is incorporated in ahousing 9601. The display portion 9603 can display images. Here, thehousing 9601 is supported by a stand 9605.

The television device 9600 can be operated with an operation switch ofthe housing 9601 or a separate remote control 9610. Channels can beswitched and volume can be controlled with operation keys 9609 of theremote control 9610, whereby an image displayed on the display portion9603 can be controlled. Moreover, the remote control 9610 may beprovided with a display portion 9607 for displaying data outputted fromthe remote control 910.

Note that the television device 900 is provided with a receiver, amodem, and the like. With the receiver, general TV broadcasts can bereceived. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (e.g., between a sender and areceiver or between receivers) information communication can beperformed.

FIG. 21B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displayimage data taken with a digital camera or the like and function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalconnectable to a variety of cables such as a USB cable), a storagemedium insertion portion, and the like. Although these components may beprovided on the same surface as the display portion, it is preferable toprovide them on the side surface or the back surface for designaesthetics. For example, storage medium storing image data taken with adigital camera are inserted into the storage medium insertion portion ofthe digital photo frame, and the data are loaded, whereby the image canbe displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. Through wireless communication, desired image data canbe loaded to be displayed.

FIG. 22A illustrates a portable game console including two housings, ahousing 9881 and a housing 9891 which are jointed with a joint portion9893 so that the portable game console can be opened or folded. Adisplay portion 9882 and a display portion 9883 are incorporated in thehousing 9881 and the housing 9891, respectively. In addition, theportable game console illustrated in FIG. 22A is provided with a speakerportion 9884, a storage medium insertion portion 9886, an LED lamp 9890,input means (operation keys 9885, a connection terminal 9887, a sensor9888 (having the function of measuring force, displacement, position,speed, acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), and amicrophone 9889), and the like. Needless to say, the structure of theportable game console is not limited to the above and other structuresprovided with at least the semiconductor device disclosed in thisspecification can be employed. The portable game console may include anadditional accessory as appropriate. The portable game consoleillustrated in FIG. 22A has the function of reading a program or datastored in a storage medium to display it on the display portion, and thefunction of sharing information with another portable game console bywireless communication. Note that the function of the portable gameconsole illustrated in FIG. 22A is not limited to those described above,and the portable game console can have a variety of functions.

FIG. 22B illustrates an example of a slot machine 9900 which is alarge-sized game machine. In the slot machine 9900, a display portion9903 is incorporated in a housing 9901. In addition, the slot machine9900 includes an operation means such as a start lever or a stop switch,a coin slot, a speaker, and the like. Needless to say, the structure ofthe slot machine 9900 is not limited to the above and other structuresprovided with at least a semiconductor device disclosed in thisspecification may be employed. The slot machine 9900 may include anadditional accessory as appropriate.

FIG. 23A is a perspective view illustrating an example of a portablecomputer.

In the portable computer illustrated in FIG. 23A a top housing 9301having a display portion 9303 and a bottom housing 9302 having akeyboard 9304 can overlap with each other by closing a hinge unit whichconnects the top housing 9301 and the bottom housing 9302. The portablecomputer illustrated in FIG. 23A is conveniently carried. In the case ofusing the keyboard for input of data, the hinge unit is opened so that auser can input data looking at the display portion 9303.

The bottom housing 9302 includes a pointing device 9306 with which inputcan be performed, in addition to the keyboard 9304. When the displayportion 9303 is a touch screen, the user can input data by touching partof the display portion. The bottom housing 9302 includes an arithmeticfunction portion such as a CPU or hard disk. In addition, the bottomhousing 9302 includes another device, for example, an externalconnection port 9305 into which a communication cable conformable tocommunication standards of a USB is inserted.

The top housing 9301 further includes a display portion 9307 which canbe stored in the top housing 9301 by being slid therein. With thedisplay portion 9307, a large display screen can be realized. Inaddition, the user can adjust the angle of a screen of the stowabledisplay portion 9307 which can be stored. If the display portion 9307which can be stored is a touch screen, the user can input data bytouching part of the display portion 9307 which can be stored.

The display portion 9303 or the display portion 9307 which can be storedis formed using an image display device such as a liquid crystal displaypanel or a light-emitting display panel using an organic light-emittingelement, an inorganic light-emitting element, or the like.

In addition, the portable computer illustrated in FIG. 23A can beprovided with a receiver and the like and can receive a TV broadcast todisplay an image on the display portion. The user can watch a TVbroadcast with the whole screen of the display portion 9307 by slidingand exposing the display portion 9307 and adjusting the angle thereof,with the hinge unit which connects the top housing 9301 and the bottomhousing 9302 closed. In this case, the hinge unit is not opened anddisplay is not performed on the display portion 9303. In addition, startup of only a circuit for displaying a TV broadcast is performed Thus,power consumption can be minimized, which is advantageous for theportable computer whose battery capacity is limited.

FIG. 23B is a perspective view of an example of a mobile phone that theuser can wear on the wrist like a wristwatch.

This mobile phone is formed with a main body which includes acommunication device including at least a telephone function, andbattery; a band portion 9204 which enables the main body to be worn onthe wrist; an adjusting portion 9205 for adjusting the band portion tofit the wrist: a display portion 9201; a speaker 9207; and a microphone9208.

In addition, the main body includes operation switches 9203. Theoperation switches 9203 can serve, for example, as a power switch, aswitch for changing display, or a switch for instruction to start takingimages. The operation switches 9203 may be a switch for starting aprogram for the Internet when the switch is pushed. The operationswitches 9203 can be configured to have respective functions.

The user can input data into this mobile phone by touching the displayportion 9201 with a finger or an input pen, controlling the operationswitches 9203, or inputting voice into the microphone 920. In FIG. 23B,display buttons 9202 are displayed on the display portion 9201. The usercan input data by touching the display buttons 9202 with a finger or thelike.

Further, the main body includes a camera portion 9206 including an imagecapturing means having the function of converting an image of an object,which is formed through a camera lens, to an electronic image signal.Note that the camera portion is not necessarily provided.

The mobile phone illustrated in FIG. 23B is provided with a receiver ofa TV broadcast and the like, and can display an image on the displayportion 9201 by receiving the TV broadcast. In addition, the mobilephone is provided with a storage device such as a memory, and can recordthe TV broadcast in the memory. The mobile phone illustrated in FIG. 23Bmay have the function of collecting location information, such as GPS.

An image display device such as a liquid crystal display panel or alight-emitting display panel using an organic or inorganiclight-emitting element is used as the display portion 9201. The mobilephone illustrated in FIG. 23B is compact and lightweight and thus haslimited battery capacity. For the above reason, a panel which can bedriven with low power consumption is preferably used as a display devicefor the display portion 9201.

Note that although FIG. 23B illustrates the electronic device which isworn on the wrist, this embodiment is not limited thereto as long as anelectronic device is portable.

Embodiment 14

In this embodiment, as one mode of a semiconductor device, examples ofdisplay devices each including the thin film transistor described in anyof Embodiments 1, 2, 5, and 6 will be described with reference to FIG.24 to FIG. 35. In this embodiment, examples of liquid crystal displaydevices in each of which a liquid crystal element is used as a displayelement will be described with reference to FIG. 24 to FIG. 35. The thinfilm transistor described in any of Embodiments 1, 2, 5, and 6 can beused as TFTs 628 and 629 used for the liquid crystal display devices inFIG. 24 to FIG. 35. The TFTs 62K and 629 can be manufactured through aprocess similar to that described in any of Embodiments 1, 2, 5, and 6and have excellent electric characteristics and high reliability. TheTFT 628 and the TFT 629 include a channel protective layer 608 and achannel protective layer 611, respectively, and are bottom gate thinfilm transistors including microcrystalline semiconductor films aschannel formation regions.

First, a vertical alignment (VA) liquid crystal display device isdescribed. The VA mode of a liquid crystal display device is a kind ofmode in which alignment of liquid crystal molecules of a liquid crystaldisplay panel is controlled. In the VA liquid crystal display device,liquid crystal molecules are aligned in a vertical direction withrespect to a panel surface when no voltage is applied. In thisembodiment, in particular, a pixel is divided into some regions(subpixels), and molecules are aligned in different directions in theirrespective regions. This is referred to as multi-domain or multi-domaindesign. Liquid crystal display devices of the multi-domain design willbe described below.

FIG. 25 and FIG. 26 illustrate a pixel electrode and a counterelectrode, respectively. FIG. 25 is a plan view showing the substrateside where the pixel electrode is formed. FIG. 24 illustrates across-sectional structure taken along section line E-F in FIG. 25. FIG.26 is a plan view showing the substrate side where the counter electrodeis formed. Description below will be made with reference to thosedrawings.

In FIG. 24, a substrate 600 over which a TFT 628, a pixel electrode 624connected to the TFT 628, and a storage capacitor portion 630 are formedand a counter substrate 601 on which a counter electrode 640 and thelike are provided overlap with each other, and liquid crystal isinjected between the substrate 600 and the counter substrate 601.

At the position where the counter substrate 601 is provided with aspacer, a first coloring film, a second coloring film, a third coloringfilm, and the counter electrode 640 are formed. With this structure, theheight of the projection 644 for controlling orientation of liquidcrystals is made different from that of the spacer. An alignment film648 is formed over the pixel electrode 624, and an alignment film 646 issimilarly formed on the counter electrode 640. A liquid crystal layer650 is formed between the substrate 600 and the counter substrate 601.

A columnar spacer may be used, or bead spacers may be dispersed. Whenthe spacer has a light-transmitting property, it may be formed over thepixel electrode 624 formed over the substrate 600.

The TFT 628, the pixel electrode 624 connected to the TFT 628, and thestorage capacitor portion 630 are formed over the substrate 600. Thepixel electrode 624 is connected to a wiring 618 through a contact hole623 which penetrates an insulating film 620 covering the TFT 628, awiring 616, and the storage capacitor portion 630 and also penetrates athird insulating film 622 covering the insulating film 620. Theinsulating film 620 is a stack of an insulating layer and a protectiveinsulating layer. The insulating layer in contact with the semiconductorlayer is a silicon oxide film formed by a sputtering method, and theprotective insulating layer over the insulating layer is a siliconnitride film formed by a sputtering method. Note that in FIG. 24, theinsulating film 620 which is a stacked layer is illustrated as a singlelayer for simplification. As the TFT 620, the thin film transistor shownin any of Embodiments 1, 2, 5, and 6 can be used as appropriate.Further, the storage capacitor portion 630 includes: a first capacitorwiring 604 which is formed at the same time as a gate wiring 602 of theTFT 628; a gate insulating film 606; and a second capacitor wiring 617which is formed at the same time as the wiring 616 and the wiring 618.

The pixel electrode 624, the liquid crystal layer 650, and the counterelectrode 640 overlap with each other, whereby a liquid crystal elementis formed.

FIG. 25 illustrates a planar structure over the substrate 600. The pixelelectrode 624 is formed using the material given in Embodiment 1. Thepixel electrode 624 is provided with slits 625. The slits 625 areprovided for controlling the alignment of the liquid crystals.

A TFT 629, a pixel electrode 626 connected to the TFT 629, and a storagecapacitor portion 631 which are illustrated in FIG. 25 can be formed ina similar manner to the TFT 628, the pixel electrode 624, and thestorage capacitor portion 630, respectively. Both the TFTs 628 and 629are connected to the wiring 616. One pixel of this liquid crystaldisplay panel includes the pixel electrode layers 624 and 626. The pixelelectrode layers 624 and 626 are each included in a sub pixel.

FIG. 26 illustrates a structure of the counter substrate side. Thecounter electrode 640 is formed over the light blocking film 632. Thecounter electrode 640 is preferably formed using a material similar tothat of the pixel electrode 624. The protrusions 644 which control thealignment of the liquid crystals are formed on the counter electrode640.

FIG. 27 illustrates an equivalent circuit of this pixel structure. Boththe TFTs 628 and 629 are connected to the gate wiring 602 and the wiring616. In that case, when potentials of the capacitor wiring 604 and acapacitor wiring 605 are different from each other, operations of liquidcrystal elements 651 and 652 can be different from each other. In otherwords, the alignment of the liquid crystals is precisely controlled anda viewing angle is increased by separate control of potentials of thecapacitor wirings 604 and 605.

When voltage is applied to the pixel electrode 624 provided with theslits 625, a distorted electric field (an oblique electric field) isgenerated in the vicinity of the slits 625. The protrusions 644 on thecounter substrate 601 side and the slits 625 are alternately arranged sothat the oblique electric field is effectively generated to control thealignment of the liquid crystals, whereby the direction of the alignmentof the liquid crystals varies depending on the location. In other words,a viewing angle of the liquid crystal display panel is increased by useof such a multi-domain configuration.

Next, a VA liquid crystal display device, which is different from theabove-described device, will be described with reference to FIG. 28 toFIG. 31.

FIG. 28 and FIG. 29 illustrate a pixel structure of a VA liquid crystaldisplay panel. FIG. 29 is a plan view of the substrate 600. FIG. 28illustrates a cross-sectional structure taken along section line Y-Z inFIG. 29. Description below will be given with reference to both thedrawings.

In this pixel structure, a plurality of pixel electrodes are provided inone pixel, and a TFT is connected to each of the pixel electrodes. Theplurality of TFTs are driven by different gate signals. In other words,signals applied to individual pixel electrodes in a multi-domain pixelare controlled independently of each other.

In the contact hole 623, the pixel electrode 624 is connected to the TFT628 through the wiring 618. In a contact hole 627, the pixel electrode626 is connected to the TFT 629 through a wiring 619. The gate wiring602 of the TFT 628 is separated from a gate wiring 603 of the TFT 629 sothat different gate signals can be supplied. On the other hand, a wiring616 serving as a data line is shared by the TFTs 628 and 629. As each ofthe TFTs 628 and 629, the thin film transistor described in Embodiments1, 2, 5, and 6 can be used as appropriate. Further, a capacitor wiring690 is provided. The insulating film 620 is a stack of an insulatinglayer and a protective insulating layer. The insulating layer in contactwith the semiconductor layer is a silicon oxide film formed by asputtering method, and the protective insulating layer over theinsulating layer is a silicon nitride film formed by a sputteringmethod. Note that in FIG. 28, the insulating film 620 which is a stackedlayer is illustrated as a single layer for simplification.

The shape of the pixel electrode 624 is different from that of the pixelelectrode 626, and the pixel electrode layers are separated by slits625. The pixel electrode 626 is formed so as to surround the pixelelectrode 624 which has a V shape. Timing of voltage application to thepixel electrode layers 624 and 626 are made to be different by the TFTs628 and 629, whereby alignment of liquid crystals is controlled. FIG. 31illustrates an equivalent circuit of this pixel structure. The TFT 628is connected to the gate wiring 602, and the TFT 629 is connected to thegate wiring 603. When different gate signals are supplied to the gatewirings 602 and 603, operation timing of the TFTs 628 and 629 can bedifferent.

The counter substrate 601 is provided with the second coloring film 636and the counter electrode 640. A planarization film 637 is formedbetween the second coloring film 636 and the counter electrode 640 toprevent alignment disorder of the liquid crystals. FIG. 30 illustrates astructure of the counter substrate side. The counter electrode 640 is anelectrode shared by different pixels and has slits 641. The slits 641and the slits 625 on the pixel electrode 624 and 626 sides arealternately arranged with each other so that an oblique electric fieldis effectively generated, whereby the alignment of the liquid crystalscan be controlled. Accordingly, the alignment of the liquid crystals canvary depending on the location, which leads to a wider viewing angle.

The pixel electrode 624, the liquid crystal layer 650, and the counterelectrode 640 overlap with each other, so that a first liquid crystalelement is formed. Further, the pixel electrode 626, the liquid crystallayer 650, and the counter electrode 640 overlap with each other, sothat a second liquid crystal element is formed. Furthermore, themulti-domain structure is employed in which the first liquid crystalelement and the second liquid crystal element are provided for onepixel.

Next, a liquid crystal display device in a horizontal electric fieldmode will be described. In the horizontal electric field mode, anelectric field in a horizontal direction is applied to liquid crystalmolecules in a cell, whereby liquid crystals are driven to express agray scale. With this method, a viewing angle can be increased to about180°. A liquid crystal display device in the horizontal electric fieldmode will be described below

In FIG. 32, the substrate 600 over which the TFT 628, and the pixelelectrode 624 connected to the TFT 62K, are formed overlaps with thecounter substrate 601, and liquid crystals are injected between thesubstrate 600 and the counter substrate 601. The counter substrate 601is provided with the second coloring film 636, the planarization film637, and the like. The pixel electrode is provided for the substrate600, and not for the counter substrate 601. A liquid crystal layer 650is formed between the substrate 600 and the counter substrate 601.

A first pixel electrode 607 and the capacitor wiring 604 connected tothe first pixel electrode 607, and the TFT 628 described in any ofEmbodiments 1, 2, 5, and 6 are formed over the substrate 600. The firstpixel electrode 67 can be formed of a material similar to that of thepixel electrode layer 427 described in Embodiment 1. Further, the firstpixel electrode 607 is formed in a shape compartmentalized roughly inaccordance with a pixel shape. Note that the gate insulating film 606 isformed over the first pixel electrode 607 and the capacitor wiring 604.

The wirings 616 and 618 of the TFT 628 are formed over the gateinsulating film 606. The wiring 616 is a data line through which a videosignal travels and is a wiring extending in one direction in a liquidcrystal display panel. The wiring 616 is also connected to a sourceregion or a drain region of the TFT 628, and serves as one of source anddrain electrodes. The wiring 618 serves as the other of the source anddrain electrodes and is connected to the second pixel electrode 624.

The second insulating film 620 is formed over the wirings 616 and 61 g.The insulating film 620 is a stack of an insulating layer and aprotective insulating layer. The insulating layer in contact with thesemiconductor layer is a silicon oxide film formed by a sputteringmethod, and the protective insulating layer over the insulating layer isa silicon nitride film formed by a sputtering method. Note that in FIG.32, the insulating film 620 which is a stacked layer is illustrated as asingle layer for simplification. Further, the second pixel electrode 624that is connected to the wiring 618 through the contact hole formed inthe insulating film 620 is formed over the insulating film 620. Thepixel electrode 624 is formed using the same material as the pixelelectrode layer 427 described in Embodiment 1.

In such a manner, the TFT 628 and the pixel electrode 624 connected tothe TFT 628 are formed over the substrate 600. Note that a storagecapacitor is formed with the pixel electrode 607 and the pixel electrode624.

FIG. 33 is a plan view illustrating a structure of the pixel electrode.FIG. 32 illustrates a cross-sectional structure taken along section lineO-P in FIG. 33. The pixel electrode 624 is provided with slits 625. Theslits 625 are provided for controlling alignment of liquid crystals. Inthat case, an electric field is generated between the pixel electrode607 and the pixel electrode 624. The thickness of the gate insulatingfilm 606 formed between the pixel electrode 607 and the pixel electrode624 is 50 nm to 200 nm, which is much smaller than the thickness of theliquid crystal layer which is 2 μm to 10 μm. Thus, an electric field isgenerated substantially in parallel (in a horizontal direction) to thesubstrate 600. The alignment of the liquid crystals is controlled withthis electric field. Liquid crystal molecules are horizontally rotatedwith the use of the electric field in the direction substantiallyparallel to the substrate. In that case, the liquid crystal moleculesare horizontally aligned in any state, and thus the contrast or the likeis less influenced by the viewing angle, which leads to a wider viewingangle. In addition, since both the pixel electrode 607 and the pixelelectrode 624 are light-transmitting electrodes, an aperture ratio canbe improved.

Next, a different example of the liquid crystal display device in thehorizontal electric field mode will be described.

FIG. 34 and FIG. 35 illustrate a pixel structure of a liquid crystaldisplay device in an IPS mode. FIG. 35 is a plan view. FIG. 34illustrates a cross-sectional structure taken along section line V-W inFIG. 35. Description below will be given with reference to both thedrawings.

In FIG. 34, the substrate 60Q over which the TFT 628 and the secondpixel electrode 624 connected to the TFT 628 are formed overlaps withthe counter substrate 601, and liquid crystals are injected between thesubstrate 600 and the counter substrate 601. The counter substrate 601is provided with the second coloring film 636, the planarization film637, and the like. The pixel electrode 624 is provided on the side ofsubstrate 600, and not on the side of the counter substrate 601. Theliquid crystal layer 650 is formed between the substrate 6) and thecounter substrate 601.

A common potential line 609 and the TFT 628 described in any ofEmbodiments 1, 2, 5, and 6 are formed over the substrate 600. The commonpotential line 609 can be formed at the same time as the gate wiring 602of the TFT 628.

The wirings 616 and 618 of the TFT 628 are formed over the gateinsulating film 606. The wiring 616 is a data line through which a videosignal travels and is a wiring extending m one direction in a liquidcrystal display panel. The wiring 616 is also connected to the sourceregion or the drain region of the TFT 628, and serves as one of sourceand drain electrodes. The wiring 618 serves as the other of the sourceand drain electrodes and is connected to the pixel electrode 624.

The second insulating film 620 is formed over the wirings 616 and 618.Further, the pixel electrode 624 that is connected to the wiring 618 inthe contact hole formed in the insulating film 620 is formed over theinsulating film 620. The insulating film 620 is a stack of an insulatinglayer and a protective insulating layer. The insulating layer in contactwith the semiconductor layer is a silicon oxide film formed by asputtering method, and the protective insulating layer over theinsulating layer is a silicon nitride film formed by a sputteringmethod. Note that in the drawing, the insulating film 620 which is astacked layer is illustrated as a single layer for simplification. Thepixel electrode 624 is formed using the same material as the pixelelectrode layer 427 described in Embodiment 1. Note that as shown inFIG. 35, the pixel electrode 624 is formed so as to generate ahorizontal electric field with a comb-shaped electrode which is formedat the same time as the common potential line 609. Further, the pixelelectrode layer 624 is formed so that comb-teeth portions of the pixelelectrode layer 624 and those of the comb-like electrode that is formedat the same time as the common potential line 609 are alternatelyarranged.

The alignment of the liquid crystals is controlled by an electric fieldgenerated due to a potential applied to the pixel electrode 624 and apotential of the common potential line 609. Liquid crystal molecules arehorizontally rotated with the use of the electric field in the directionsubstantially parallel to the substrate. In that case, the liquidcrystal molecules are horizontally aligned in any state, and thus thecontrast or the like is less influenced by the viewing angle, whichleads to a wider viewing angle.

In such a manner, the TFT 628 and the pixel electrode 624 connected tothe TFT 628 are formed over the substrate 600. The gate insulating film606 is provided between the common potential line 609 and a capacitorelectrode 615 to form a storage capacitor. The capacitor electrode 615and the pixel electrode 624 are connected to each other through acontact hole 633.

Through the above process, a liquid crystal display device can bemanufactured as a display device. The liquid crystal display devices ofthis embodiment are highly reliable liquid crystal display devices.

This application is based on Japanese Patent Application serial no.2009-180077 filed with the Japan Patent Office on Jul. 31, 2009, theentire contents of which are hereby incorporated by reference.

1. (canceled)
 2. A display device comprising: a transistor comprising: afirst oxide semiconductor layer; an oxide insulating layer comprising afirst region in contact with a peripheral portion of the first oxidesemiconductor layer; and a first conductive layer configured to functionas one of a source electrode and a drain electrode of the transistor; apixel electrode electrically connected to the transistor; and acapacitor electrically connected to the transistor, comprising: a secondoxide semiconductor layer; a second conductive layer below the secondoxide semiconductor layer; and a third conductive layer above the secondoxide semiconductor layer, wherein the one of the source electrode andthe drain electrode of the transistor comprises a second region incontact with a top surface of the oxide insulating layer and iselectrically connected to the first oxide semiconductor layer through acontact hole provided in the oxide insulating layer, wherein the thirdconductive layer comprises a third region in contact with the topsurface of the oxide insulating layer, wherein the oxide insulatinglayer comprises a fourth region in contact with a peripheral portion ofthe second oxide semiconductor layer, and wherein the pixel electrodecomprises a fifth region overlapping with the transistor and a sixthregion overlapping with the capacitor.
 3. The display device accordingto claim 2, further comprising: an inorganic insulating layer over theoxide insulating layer and the first conductive layer; and aplanarization insulation layer over the inorganic insulating layer,wherein the pixel electrode is over the planarization insulation layer.4. The display device according to claim 2, wherein each of the firstoxide semiconductor layer and the second oxide semiconductor layercomprise In, Ga, and Zn.
 5. A display device comprising: a transistorcomprising: a gate electrode; a first oxide semiconductor layer; anoxide insulating layer comprising a first region in contact with aperipheral portion of the first oxide semiconductor layer; and a firstconductive layer configured to function as one of a source electrode anda drain electrode of the transistor; a pixel electrode electricallyconnected to the transistor; and a capacitor electrically connected tothe transistor, comprising: a second oxide semiconductor layer; a secondconductive layer below the second oxide semiconductor layer; and a thirdconductive layer above the second oxide semiconductor layer, wherein thegate electrode of the transistor and the second oxide semiconductorlayer comprise the same material, wherein the one of the sourceelectrode and the drain electrode of the transistor comprises a secondregion in contact with a top surface of the oxide insulating layer andis electrically connected to the first oxide semiconductor layer througha contact hole provided in the oxide insulating layer, wherein the thirdconductive layer comprises a third region in contact with the topsurface of the oxide insulating layer, wherein the oxide insulatinglayer comprises a fourth region in contact with a peripheral portion ofthe second oxide semiconductor layer, and wherein the pixel electrodecomprises a fifth region overlapping with the transistor and a sixthregion overlapping with the capacitor.
 6. The display device accordingto claim 5, further comprising: an inorganic insulating layer over theoxide insulating layer and the first conductive layer; and aplanarization insulation layer over the inorganic insulating layer,wherein the pixel electrode is over the planarization insulation layer.7. The display device according to claim 5, wherein each of the firstoxide semiconductor layer and the second oxide semiconductor layercomprise In, Ga, and Zn.